Sometimes, after an indeterminate amount of time with firefox open, some UI elements of firefox stop working. Clicking on the bookmarks button, or the 'open the menu' button, or the 'downloads' button causes the button to appear to depress and release, but nothing shows up. At the same time, scrolling the web page with the mousewheel stops working, or only works 1 out of every 5 times. However, buttons that don't open new panels, like the back button, work. The address bar works (but does not show the suggestions drop-down, only autocompletions within the address bar text). Hyperlinks work, and I can browse and load pages like normal. Scrolling with PgUp and PgDn reliably works. Flash animations play, HTML5 videos play. Safe mode does /not/ prevent it from happening. I cannot figure out a way to make it happen on-demand, it seems to be able to happen any time after I start firefox, but only rarely, maybe a few times a week. It seems to happen more often if I have many tabs open, though I only have 22 tabs open at the moment, in two windows, and it's happening. Closing tabs, or closing one window, does not seem to help. Restarting the browser fixes it, though it happens again after a while. Firefox is currently using ~700 MB of RAM, and it's fluctuating by about 20 MB either way over the course of a few seconds as I type this. I have approximately 16 GB of RAM available. If I close firefox completely, and try to reopen it immediately, it sometimes says 'Firefox is running but not responding', but it didn't happen when I just tried it then. I am technically proficient and able to run any linux commands if that helps with debugging. I admit that I'm not using the current version of firefox right at the moment, but it's only a month or so out of date, and the problem has been happening on-and-off for years, so it's unlikely that a newer version would fix it (I will update after I submit this, and if the problem happens again I'll let you know, but it's difficult to reproduce).