I don't have that issue with my Distro nor Mozilla version of Firefox. see screenshot

Are you running any themes? Did you try with Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf Try downloading Firefox from Mozilla. Download, unzip, and run firefox-bin from the folder and see if you have the same issue. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Thanks for you guide. I install firefox via apt install, use the default settings with rename `~/.mozilla` data dir, it should not to be the problem with themes or settings, and should be reappear by anyone work in ubuntu 20.04 LTS.