I'm having an issue whereby none of the UI popups in Firefox will stay on the screen for more than a few milliseconds. Any popup - a menubar menu, right click menu, autocomplete suggestion in the URL bar, the "would you like to remember this password" popup after logging in - they all flash on the screen then disappear immediately. If you hold the ESC key down on your keyboard and click around the UI with the key held down, then it will give you a similar experience to what I am seeing. When I load Firefox it works fine for a few hours then randomly gets into this state. When it happens, every other program on the system is fine, it is only Firefox that cannot keep any popup windows visible. It affects all Firefox windows the same. The menus won't open even if I use Alt+F for example to open them. I tried loading a second instance of Firefox with --safe-mode --ProfileManager --new-instance and after accepting the warning about being in safe mode, the browser came up with the same problem. So even in safe mode the menus won't open. I've used the "xev" command to confirm that there are no keys being held down on the keyboard, and exiting Firefox and reopening it solves the problem for another few hours, but it's a pain having to restart the browser all the time. I do a lot of web dev work in private windows to maintain multiple sessions to websites, and restarting Firefox loses all those windows and sessions which is a real pain. Weirdly after restarting Firefox just now and having the problem go away, I then tried running safe mode again as before in a second instance, and that time the safe mode window could not show any menus but the normal instance was working correctly. I pressed Ctrl+Shift+A to go to the add-on window (since there is no other way to get there if the menus won't open) and confirmed that all addons were temporarily disabled, so the browser was definitely running in safe mode when it was unable to keep any popup UI elements visible for more than a fraction of a second. I've had this issue on and off since Firefox 3, now running FF 63 but I've never been able to figure out what's going on. It has only ever happened to Firefox and not any other program on the system. It's also happened on four different computers, although they were all running Arch Linux. It doesn't appear to be a graphics problem as one of these systems has an nVidia Quadro with the closed-source driver, and the others are using Intel integrated graphics with the open-source driver. I'm not running a compositing window manager either. It *seems* like moving/resizing the window could be what triggers it but I'm not certain. Does anyone have any ideas what could cause this or how to narrow it down any further? I thought Firefox used GTK for managing windows, has that changed? None of my other GTK apps have any issues like this.