The problem is with GTK function stubs being declared in the wrong sections in widget/gtk/mozgtk/mozgtk.c. Another recent upstream change did that too: https://phabricator.services.mozilla.com/D82804.
And this patch fixes it: https://bazaar.launchpad.net/~mozillateam/firefox/firefox.xenial/revision/1450 ** Changed in: firefox (Ubuntu) Status: Triaged => Fix Committed -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to firefox in Ubuntu. https://bugs.launchpad.net/bugs/1893021 Title: Flash plugin symbol lookup error with firefox 80.0+build2 on xenial (gtk_window_set_titlebar) Status in firefox package in Ubuntu: Fix Committed Bug description: When testing firefox 80.0+build2 on xenial (from https://launchpad.net /~ubuntu-mozilla-security/+archive/ubuntu/ppa/+packages), I'm observing that the flash plugin (installed by flashplugin-installer) doesn't work. This appears to affect only xenial, flash is working in the bionic and focal builds. This is what I'm seeing when launching firefox from a terminal: /usr/lib/firefox/plugin-container: symbol lookup error: /usr/lib/firefox/libxul.so: undefined symbol: gtk_window_set_titlebar This is most likely caused by this recent upstream change: https://phabricator.services.mozilla.com/D84623 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1893021/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp