I don't think it's just that bug. I have a userstyle to mark the displayed element, so I sometimes see when something is grabing the focus, and neither tab nor ESC or Alt-l helps in those cases. But after some reseach I found out F6 can help there. Till now I thought it's just the same as Alt-l and only focus the addressbar, but turns out it can also unfocus it and switch focus between the different areas of firefox. I tested it in the last hour and it really worked in several cases where I was stuck in focus-hell.