--- a/netwerk/base/nsFileStreams.cpp
+++ b/netwerk/base/nsFileStreams.cpp
@@ -800,21 +800,28 @@ nsAtomicFileOutputStream::DoOpen()
 
         // XP_UNIX ignores SetFollowLinks(), so we have to normalize.
         if (mTargetFileExists) {
             tempResult->Normalize();
         }
     }
 
     if (NS_SUCCEEDED(rv) && mTargetFileExists) {
+        // Abort if |file| is not writable; it won't work as an output stream.
+        bool isWritable;
+        if (NS_SUCCEEDED(file->IsWritable(&isWritable)) && !isWritable) {
+            return NS_ERROR_FILE_ACCESS_DENIED;
+        }
+
         uint32_t origPerm;
         if (NS_FAILED(file->GetPermissions(&origPerm))) {
             NS_ERROR("Can't get permissions of target file");
             origPerm = mOpenParams.perm;
         }
+
         // XXX What if |perm| is more restrictive then |origPerm|?
         // This leaves the user supplied permissions as they were.
         rv = tempResult->CreateUnique(nsIFile::NORMAL_FILE_TYPE, origPerm);
     }
     if (NS_SUCCEEDED(rv)) {
         // nsFileOutputStream::DoOpen will work on the temporary file, so we
         // prepare it and place it in mOpenParams.localFile.
         mOpenParams.localFile = tempResult;