Another issue I have had recently is where I click a thread (Usually I'm opening tons at once when I first log on that day, all my notificiations, so maybe its related to loading tons of pages at once) and it has youtube videos and/or pictures in it, the youtube overlay "API" doesn't load so the video is just the thumbnail image and if I click it nothing happens, and the images aren't clickable either. I refresh the page and it usually fixes it but sometimes I have to do it multiple times.
May also be FF issue as its always had weird issues since they moved over to this "every tab is its own instance" method, which is great most of the time when one tab crashes but sometimes I get weird visual bugs like I click on a tab and the whole FF window is a solid light blue box, with no menu bar or anything.