I just checked it with a fresh install of Xubuntu 20.04.01 on a different machine, and I cannot reproduce the problem. I went back and checked it again on my main computer, and the bug went away! The last crash was two days ago on 2020-08-27, and now it doesn't crash anymore when I logout or shutdown. It seems that it has been fixed in firefox 80.0.
apt-cache policy firefox firefox: Installed: 80.0+build2-0ubuntu0.20.04.1 Candidate: 80.0+build2-0ubuntu0.20.04.1 Version table: *** 80.0+build2-0ubuntu0.20.04.1 500 500 http://mirror.csclub.uwaterloo.ca/ubuntu focal-updates/main amd64 Packages 500 http://mirror.csclub.uwaterloo.ca/ubuntu focal-security/main amd64 Packages 100 /var/lib/dpkg/status 75.0+build3-0ubuntu1 500 500 http://mirror.csclub.uwaterloo.ca/ubuntu focal/main amd64 Packages -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1892373 Title: Firefox crashes on logout To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1892373/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs