diff --git a/widget/android/PrefsHelper.h b/widget/android/PrefsHelper.h
index fa7a31cffb0..afe47ffb695 100644
--- a/widget/android/PrefsHelper.h
+++ b/widget/android/PrefsHelper.h
@@ -9,70 +9,308 @@
 #include "GeneratedJNINatives.h"
 #include "MainThreadUtils.h"
 #include "nsAppShell.h"
+#include "nsCOMPtr.h"
+#include "nsVariant.h"
 
+#include "mozilla/Preferences.h"
+#include "mozilla/Services.h"
 #include "mozilla/UniquePtr.h"
 
 namespace mozilla {
 
-struct PrefsHelper
-    : widget::PrefsHelper::Natives<PrefsHelper>
-    , UsesGeckoThreadProxy
+class PrefsHelper
+    : public widget::PrefsHelper::Natives<PrefsHelper>
+    , public UsesGeckoThreadProxy
 {
     PrefsHelper() = delete;
 
-    static void GetPrefsById(const jni::ClassObject::LocalRef& cls,
-                             int32_t requestId,
-                             jni::ObjectArray::Param prefNames,
-                             bool observe)
+    static bool GetVariantPref(nsIObserverService* aObsServ,
+                               nsIWritableVariant* aVariant,
+                               jni::Object::Param aPrefHandler,
+                               jni::String::LocalRef& aPrefName)
     {
-        MOZ_ASSERT(NS_IsMainThread());
-
-        nsAppShell* const appShell = nsAppShell::Get();
-        MOZ_ASSERT(appShell);
-
-        nsTArray<jni::Object::LocalRef> namesRefArray(prefNames.GetElements());
-        const size_t len = namesRefArray.Length();
-
-        // Java strings are not null-terminated, but XPCOM expects
-        // null-terminated strings, so we copy the strings to nsString to
-        // ensure null-termination.
-        nsTArray<nsString> namesStrArray(len);
-        nsTArray<const char16_t*> namesPtrArray(len);
-
-        for (size_t i = 0; i < len; i++) {
-            if (!namesRefArray[i]) {
-                namesPtrArray.AppendElement(nullptr);
-                continue;
-            }
-            namesPtrArray.AppendElement(namesStrArray.AppendElement(nsString(
-                    jni::String::Ref::From(namesRefArray[i])))->Data());
+        if (NS_FAILED(aObsServ->NotifyObservers(aVariant, "android-get-pref",
+                                                nsString(aPrefName).get()))) {
+            return false;
         }
 
-        nsIAndroidBrowserApp* browserApp = nullptr;
-        appShell->GetBrowserApp(&browserApp);
-        MOZ_ASSERT(browserApp);
+        uint16_t varType = nsIDataType::VTYPE_EMPTY;
+        if (NS_FAILED(aVariant->GetDataType(&varType))) {
+            return false;
+        }
 
-        if (observe) {
-            browserApp->ObservePreferences(
-                    requestId, len ? &namesPtrArray[0] : nullptr, len);
-        } else {
-            browserApp->GetPreferences(
-                    requestId, len ? &namesPtrArray[0] : nullptr, len);
+        int32_t type = widget::PrefsHelper::PREF_INVALID;
+        bool boolVal = false;
+        int32_t intVal = 0;
+        nsAutoString strVal;
+
+        switch (varType) {
+            case nsIDataType::VTYPE_BOOL:
+                type = widget::PrefsHelper::PREF_BOOL;
+                if (NS_FAILED(aVariant->GetAsBool(&boolVal))) {
+                    return false;
+                }
+                break;
+            case nsIDataType::VTYPE_INT32:
+                type = widget::PrefsHelper::PREF_INT;
+                if (NS_FAILED(aVariant->GetAsInt32(&intVal))) {
+                    return false;
+                }
+                break;
+            case nsIDataType::VTYPE_ASTRING:
+                type = widget::PrefsHelper::PREF_STRING;
+                if (NS_FAILED(aVariant->GetAsAString(strVal))) {
+                    return false;
+                }
+                break;
+            default:
+                return false;
+        }
+
+        const auto& jstrVal = type == widget::PrefsHelper::PREF_STRING ?
+                jni::Param<jni::String>(strVal, aPrefName.Env()) :
+                jni::Param<jni::String>(nullptr);
+
+        widget::PrefsHelper::CallPrefHandler(
+                aPrefHandler, type, aPrefName, boolVal, intVal, jstrVal);
+        return true;
+    }
+
+    static bool SetVariantPref(nsIObserverService* aObsServ,
+                               nsIWritableVariant* aVariant,
+                               jni::String::Param aPrefName,
+                               bool aFlush,
+                               int32_t aType,
+                               bool aBoolVal,
+                               int32_t aIntVal,
+                               jni::String::Param aStrVal)
+    {
+        nsresult rv = NS_ERROR_FAILURE;
+
+        switch (aType) {
+            case widget::PrefsHelper::PREF_BOOL:
+                rv = aVariant->SetAsBool(aBoolVal);
+                break;
+            case widget::PrefsHelper::PREF_INT:
+                rv = aVariant->SetAsInt32(aIntVal);
+                break;
+            case widget::PrefsHelper::PREF_STRING:
+                rv = aVariant->SetAsAString(nsString(aStrVal));
+                break;
+        }
+
+        if (NS_SUCCEEDED(rv)) {
+            rv = aObsServ->NotifyObservers(aVariant, "android-set-pref",
+                                           nsString(aPrefName).get());
+        }
+
+        uint16_t varType = nsIDataType::VTYPE_EMPTY;
+        if (NS_SUCCEEDED(rv)) {
+            rv = aVariant->GetDataType(&varType);
+        }
+
+        // We use set-to-empty to signal the pref was handled.
+        const bool handled = varType == nsIDataType::VTYPE_EMPTY;
+
+        if (NS_SUCCEEDED(rv) && handled && aFlush) {
+            rv = Preferences::GetService()->SavePrefFile(nullptr);
+        }
+
+        if (NS_SUCCEEDED(rv)) {
+            return handled;
+        }
+
+        NS_WARNING(nsPrintfCString("Failed to set pref %s",
+                                   nsCString(aPrefName).get()).get());
+        // Pretend we handled the pref.
+        return true;
+    }
+
+public:
+    static void GetPrefs(const jni::ClassObject::LocalRef& aCls,
+                         jni::ObjectArray::Param aPrefNames,
+                         jni::Object::Param aPrefHandler)
+    {
+        nsTArray<jni::Object::LocalRef> nameRefArray(aPrefNames.GetElements());
+        nsCOMPtr<nsIObserverService> obsServ;
+        nsCOMPtr<nsIWritableVariant> value;
+        nsAdoptingString strVal;
+
+        for (jni::Object::LocalRef& nameRef : nameRefArray) {
+            jni::String::LocalRef nameStr(mozilla::Move(nameRef));
+            const nsCString& name = nsCString(nameStr);
+
+            int32_t type = widget::PrefsHelper::PREF_INVALID;
+            bool boolVal = false;
+            int32_t intVal = 0;
+
+            switch (Preferences::GetType(name.get())) {
+                case nsIPrefBranch::PREF_BOOL:
+                    type = widget::PrefsHelper::PREF_BOOL;
+                    boolVal = Preferences::GetBool(name.get());
+                    break;
+
+                case nsIPrefBranch::PREF_INT:
+                    type = widget::PrefsHelper::PREF_INT;
+                    intVal = Preferences::GetInt(name.get());
+                    break;
+
+                case nsIPrefBranch::PREF_STRING:
+                    type = widget::PrefsHelper::PREF_STRING;
+                    strVal = Preferences::GetLocalizedString(name.get());
+                    if (!strVal) {
+                        strVal = Preferences::GetString(name.get());
+                    }
+                    break;
+
+                default:
+                    // Pref not found; try to find it.
+                    if (!obsServ) {
+                        obsServ = services::GetObserverService();
+                        if (!obsServ) {
+                            continue;
+                        }
+                    }
+                    if (value) {
+                        value->SetAsEmpty();
+                    } else {
+                        value = new nsVariant();
+                    }
+                    if (!GetVariantPref(obsServ, value,
+                                        aPrefHandler, nameStr)) {
+                        NS_WARNING(nsPrintfCString("Failed to get pref %s",
+                                                   name.get()).get());
+                    }
+                    continue;
+            }
+
+            const auto& jstrVal = type == widget::PrefsHelper::PREF_STRING ?
+                    jni::Param<jni::String>(strVal, aCls.Env()) :
+                    jni::Param<jni::String>(nullptr);
+
+            widget::PrefsHelper::CallPrefHandler(
+                    aPrefHandler, type, nameStr, boolVal, intVal, jstrVal);
+        }
+
+        widget::PrefsHelper::CallPrefHandler(
+                aPrefHandler, widget::PrefsHelper::PREF_FINISH,
+                nullptr, false, 0, nullptr);
+    }
+
+    static void SetPref(jni::String::Param aPrefName,
+                        bool aFlush,
+                        int32_t aType,
+                        bool aBoolVal,
+                        int32_t aIntVal,
+                        jni::String::Param aStrVal)
+    {
+        const nsCString& name = nsCString(aPrefName);
+
+        if (Preferences::GetType(name.get()) == nsIPrefBranch::PREF_INVALID) {
+            // No pref; try asking first.
+            nsCOMPtr<nsIObserverService> obsServ =
+                    services::GetObserverService();
+            nsCOMPtr<nsIWritableVariant> value = new nsVariant();
+            if (obsServ && SetVariantPref(obsServ, value, aPrefName, aFlush,
+                                          aType, aBoolVal, aIntVal, aStrVal)) {
+                return;
+            }
+        }
+
+        switch (aType) {
+            case widget::PrefsHelper::PREF_BOOL:
+                Preferences::SetBool(name.get(), aBoolVal);
+                break;
+            case widget::PrefsHelper::PREF_INT:
+                Preferences::SetInt(name.get(), aIntVal);
+                break;
+            case widget::PrefsHelper::PREF_STRING:
+                Preferences::SetString(name.get(), nsString(aStrVal));
+                break;
+            default:
+                MOZ_ASSERT(false, "Invalid pref type");
+        }
+
+        if (aFlush) {
+            Preferences::GetService()->SavePrefFile(nullptr);
         }
     }
 
-    static void RemovePrefsObserver(int32_t requestId)
+    static void AddObserver(const jni::ClassObject::LocalRef& aCls,
+                            jni::ObjectArray::Param aPrefNames,
+                            jni::Object::Param aPrefHandler,
+                            jni::ObjectArray::Param aPrefsToObserve)
     {
-        MOZ_ASSERT(NS_IsMainThread());
+        // Call observer immediately with existing pref values.
+        GetPrefs(aCls, aPrefNames, aPrefHandler);
 
+        if (!aPrefsToObserve) {
+            return;
+        }
+
+        nsTArray<jni::Object::LocalRef> nameRefArray(
+                aPrefsToObserve.GetElements());
         nsAppShell* const appShell = nsAppShell::Get();
         MOZ_ASSERT(appShell);
 
-        nsIAndroidBrowserApp* browserApp = nullptr;
-        appShell->GetBrowserApp(&browserApp);
-        MOZ_ASSERT(browserApp);
+        for (jni::Object::LocalRef& nameRef : nameRefArray) {
+            jni::String::LocalRef nameStr(mozilla::Move(nameRef));
+            MOZ_ALWAYS_TRUE(NS_SUCCEEDED(Preferences::AddStrongObserver(
+                    appShell, nsCString(nameStr).get())));
+        }
+    }
 
-        browserApp->RemovePreferenceObservers(requestId);
+    static void RemoveObserver(const jni::ClassObject::LocalRef& aCls,
+                               jni::ObjectArray::Param aPrefsToUnobserve)
+    {
+        nsTArray<jni::Object::LocalRef> nameRefArray(
+                aPrefsToUnobserve.GetElements());
+        nsAppShell* const appShell = nsAppShell::Get();
+        MOZ_ASSERT(appShell);
+
+        for (jni::Object::LocalRef& nameRef : nameRefArray) {
+            jni::String::LocalRef nameStr(mozilla::Move(nameRef));
+            MOZ_ALWAYS_TRUE(NS_SUCCEEDED(Preferences::RemoveObserver(
+                    appShell, nsCString(nameStr).get())));
+        }
+    }
+
+    static void OnPrefChange(const char16_t* aData)
+    {
+        const nsCString& name = NS_LossyConvertUTF16toASCII(aData);
+
+        int32_t type = -1;
+        bool boolVal = false;
+        int32_t intVal = false;
+        nsAdoptingString strVal;
+
+        switch (Preferences::GetType(name.get())) {
+            case nsIPrefBranch::PREF_BOOL:
+                type = widget::PrefsHelper::PREF_BOOL;
+                boolVal = Preferences::GetBool(name.get());
+                break;
+            case nsIPrefBranch::PREF_INT:
+                type = widget::PrefsHelper::PREF_INT;
+                intVal = Preferences::GetInt(name.get());
+                break;
+            case nsIPrefBranch::PREF_STRING:
+                type = widget::PrefsHelper::PREF_STRING;
+                strVal = Preferences::GetLocalizedString(name.get());
+                if (!strVal) {
+                    strVal = Preferences::GetString(name.get());
+                }
+                break;
+            default:
+                NS_WARNING(nsPrintfCString("Invalid pref %s",
+                                           name.get()).get());
+                return;
+        }
+
+        const auto& jstrVal = type == widget::PrefsHelper::PREF_STRING ?
+                jni::Param<jni::String>(strVal) :
+                jni::Param<jni::String>(nullptr);
+
+        widget::PrefsHelper::OnPrefChange(name, type, boolVal, intVal, jstrVal);
     }
 };
 
diff --git a/widget/android/jni/Refs.h b/widget/android/jni/Refs.h
index 52469c4f8ce..a954016641b 100644
--- a/widget/android/jni/Refs.h
+++ b/widget/android/jni/Refs.h
@@ -776,6 +776,11 @@ private:
     }
 
 public:
+    MOZ_IMPLICIT Type(decltype(nullptr))
+        : Ref<String>(nullptr)
+        , mEnv(nullptr)
+    {}
+
     MOZ_IMPLICIT Type(const String::Ref& ref)
         : Ref<String>(ref.Get())
         , mEnv(nullptr)
diff --git a/widget/android/nsAppShell.cpp b/widget/android/nsAppShell.cpp
index 4e085a87d6e..609bfed714d 100644
--- a/widget/android/nsAppShell.cpp
+++ b/widget/android/nsAppShell.cpp
@@ -341,6 +341,11 @@ nsAppShell::Observe(nsISupports* aSubject,
             }
         }
         removeObserver = true;
+
+    } else if (!strcmp(aTopic, "nsPref:changed")) {
+        if (jni::IsAvailable()) {
+            mozilla::PrefsHelper::OnPrefChange(aData);
+        }
     }
 
     if (removeObserver) {