Almost perfect, but has a few bugs:
1. Stops working randomly. Restarting Firefox fixes usually
2. Sometimes the framerates show up very briefly, and I have to keep re-hovering over videos to have it show up long enough to read. Restarting does not fix.

I really like this addon and hope Tim continues to support it! Maybe one day there will be some kind of auto-refresh rate switch built into windows, but until then I have to manually change it every time I watch a video. This addon makes that just a bit nicer.