I am seeing problems identical to https://support.mozilla.org/en-US/questions/1273349 When I click a menu it briefly appears and then immediately disappears. Turning on "Disable popup auto-hide" solves the issue, but this is a work around not a solution. Restarting Firefox with a clean brand new profile resolves the problem. So it seems nothing to do with any of my plugins. But after I click on a link from within emacs, the first time I open a menu it is fine. Then the 2nd time I try to open a menu it disappears immediately. There might be other conditions that do this also, but this is the most reliable way I have found to date. I believe emacs is calling firefox, by default, using "xdg-open". If I manually use this command from a command prompt to open a web page. It works fine. Huh??? I changed emacs to explicitly invoke Firefox. At first this seemed to have solved this problem. But now it has come back again. This has been ongoing for some time, over a large range of Firefox versions. Currently I can reproduce with 68.11.0 ESR and 80.0.1 Sometimes I have done something and I think I have magically resolved the issue, but then it comes back again. This is currently Debian/buster and Debian/bullseye using the i3 window manager. In the past I have also had the same issue with Debian/stretch. I have multiple monitors. I doubt any of this is relevant, but just in case. Unlike the previous referenced post I have not been able to reproduce this under OSX. Although I don't often click links from emacs either. So wonder if that is significant.