Hey Jim,

I've gone through the extension and found a few issues specifically with Firefox that may have been causing this for you. I've updated and it should be alot more stable. Additionally I've introduced an options menu where you can set any shortcut key you'd like now. All you'll have to do is go to about:addons in your address bar, select YouTube windowed fullscreen, and then the Options tab to set these.

Let me know how it goes, thanks!