I did find one bug that's been driving me nuts. Some videos don't display at all on youtube when the add-on is activated. For me it seems to be any video with a 4 in the title. The videos show up anywhere outside youtube (such as google search), but clicking on them causes Firefox to freak out and automatically go back a page.
This can be prevented by opening the video in a new tab, but you still can't see them while browsing youtube if it has a 4 in the title. This means videos like numbered Lets Plays that are numbered 39-50 or 399-500 are invisible. That's at maximum 100 videos you're missing out on.
I have to turn the add-on off every time this happens, then turn it back on again whenever youtube gives me garbage recommendations.