diff --git a/toolkit/xre/nsAppRunner.cpp b/toolkit/xre/nsAppRunner.cpp
index 02910c82807..95180f488ce 100644
--- a/toolkit/xre/nsAppRunner.cpp
+++ b/toolkit/xre/nsAppRunner.cpp
@@ -3304,14 +3304,17 @@ XREMain::XRE_mainInit(bool* aExitFlag)
   // order bit will be 1 if the key is pressed. By masking the returned short
   // with 0x8000 the result will be 0 if the key is not pressed and non-zero
   // otherwise.
-  if (GetKeyState(VK_SHIFT) & 0x8000 &&
-      !(GetKeyState(VK_CONTROL) & 0x8000) && !(GetKeyState(VK_MENU) & 0x8000)) {
+  if ((GetKeyState(VK_SHIFT) & 0x8000) &&
+      !(GetKeyState(VK_CONTROL) & 0x8000) &&
+      !(GetKeyState(VK_MENU) & 0x8000) &&
+      !EnvHasValue("MOZ_DISABLE_SAFE_MODE_KEY")) {
     gSafeMode = true;
   }
 #endif
 
 #ifdef XP_MACOSX
-  if (GetCurrentEventKeyModifiers() & optionKey)
+  if ((GetCurrentEventKeyModifiers() & optionKey) &&
+      !EnvHasValue("MOZ_DISABLE_SAFE_MODE_KEY"))
     gSafeMode = true;
 #endif