diff --git a/netwerk/base/src/nsFileStreams.cpp b/netwerk/base/src/nsFileStreams.cpp
index cfd95562365..9775122b18d 100644
--- a/netwerk/base/src/nsFileStreams.cpp
+++ b/netwerk/base/src/nsFileStreams.cpp
@@ -869,6 +869,13 @@ NS_IMETHODIMP
 nsAtomicFileOutputStream::Init(nsIFile* file, int32_t ioFlags, int32_t perm,
                              int32_t behaviorFlags)
 {
+    // While `PR_APPEND` is not supported, `-1` is used as `ioFlags` parameter
+    // in some places, and `PR_APPEND | PR_TRUNCATE` does not require appending
+    // to existing file. So, throw an exception only if `PR_APPEND` is
+    // explicitly specified without `PR_TRUNCATE`.
+    if ((ioFlags & PR_APPEND) && !(ioFlags & PR_TRUNCATE)) {
+        return NS_ERROR_INVALID_ARG;
+    }
     return nsFileOutputStream::Init(file, ioFlags, perm, behaviorFlags);
 }
 
diff --git a/netwerk/test/unit/test_safeoutputstream_append.js b/netwerk/test/unit/test_safeoutputstream_append.js
new file mode 100644
index 00000000000..a7abb06e651
--- /dev/null
+++ b/netwerk/test/unit/test_safeoutputstream_append.js
@@ -0,0 +1,42 @@
+/* atomic-file-output-stream and safe-file-output-stream should throw and
+ * exception if PR_APPEND is explicity specified without PR_TRUNCATE. */
+
+const PR_WRONLY      = 0x02;
+const PR_CREATE_FILE = 0x08;
+const PR_APPEND      = 0x10;
+const PR_TRUNCATE    = 0x20;
+
+const { Services } = Cu.import("resource://gre/modules/Services.jsm", {});
+
+function check_flag(file, contractID, flags, throws) {
+  let stream = Cc[contractID].createInstance(Ci.nsIFileOutputStream);
+
+  if (throws) {
+    /* NS_ERROR_INVALID_ARG is reported as NS_ERROR_ILLEGAL_VALUE, since they
+     * are same value. */
+    Assert.throws(() => stream.init(file, flags, 0o644, 0),
+                  /NS_ERROR_ILLEGAL_VALUE/);
+  } else {
+    stream.init(file, flags, 0o644, 0);
+    stream.close();
+  }
+}
+
+function run_test() {
+  let filename = "test.txt";
+  let file = Services.dirsvc.get("TmpD", Ci.nsIFile);
+  file.append(filename);
+
+  let tests = [
+    [PR_WRONLY | PR_CREATE_FILE | PR_APPEND | PR_TRUNCATE, false],
+    [PR_WRONLY | PR_CREATE_FILE | PR_TRUNCATE, false],
+    [PR_WRONLY | PR_CREATE_FILE | PR_APPEND, true],
+    [-1, false],
+  ];
+  for (let contractID of ["@mozilla.org/network/atomic-file-output-stream;1",
+                          "@mozilla.org/network/safe-file-output-stream;1"]) {
+    for (let [flags, throws] of tests) {
+      check_flag(file, contractID, flags, throws);
+    }
+  }
+}
diff --git a/netwerk/test/unit/xpcshell.ini b/netwerk/test/unit/xpcshell.ini
index 54df6381094..ed8ad166cb9 100644
--- a/netwerk/test/unit/xpcshell.ini
+++ b/netwerk/test/unit/xpcshell.ini
@@ -307,3 +307,4 @@ run-if = os == "win"
 # The local cert service used by this test is not currently shipped on Android
 skip-if = os == "android"
 [test_1073747.js]
+[test_safeoutputstream_append.js]