Thanks for the review. Unfortunately I wasn't able to reproduce the issue you described when I went to YouTube.

Sort of reminds me of how pressing Alt (and bringing up the application's menu) can sometimes block hotkeys though (fixable by clicking anywhere on the page and trying again). If you've freshly installed or enabled the add-on this (and most) add-ons won't run until the page is refreshed as well.

If those don't help, then I'll probably need more information to try and help you with your issue, like what website and what other add-ons or settings that might be different than default Firefox on Windows. You can email me with the Support email link and I'll be happy to help!