firefox: add patch to fix the thread name

instead of MainThread, use firefox as thread name
this is just a minor issue, so no rebuild

srcpkgs/firefox/patches/fix-threadname.patch

@@ -0,0 +1,13 @@
+this reverts https://hg.mozilla.org/mozilla-central/rev/9f8a59996994
+
+--- toolkit/xre/nsAppRunner.cpp	2020-07-21 00:49:52.000000000 +0200
++++ toolkit/xre/nsAppRunner.cpp	2020-08-17 21:12:47.688908716 +0200
+@@ -4741,8 +4741,6 @@
+   CodeCoverageHandler::Init();
+ #endif
+ 
+-  NS_SetCurrentThreadName("MainThread");
+-
+   AUTO_BASE_PROFILER_LABEL("XREMain::XRE_main (around Gecko Profiler)", OTHER);
+   AUTO_PROFILER_INIT;
+   AUTO_PROFILER_LABEL("XREMain::XRE_main", OTHER);