BB 43386: Use Firefox in the UA in RFP-exempt request.

XHR requests initiated by extensions are exempt from RFP.
Therefore, they report the actual app name, instead of Firefox, and the
actual Firefox minor version.

This happens whenever the app name has been customized and does not
match a hardcoded "Firefox".

netwerk/protocol/http/nsHttpHandler.cpp

@@ -953,7 +953,7 @@ void nsHttpHandler::BuildUserAgent() {
   mUserAgent += '/';
   mUserAgent += mProductSub;
 
-  bool isFirefox = mAppName.EqualsLiteral("Firefox");
+  bool isFirefox = true;
   if (isFirefox || mCompatFirefoxEnabled) {
     // "Firefox/x.y" (compatibility) app token
     mUserAgent += ' ';