Comment 28 for bug 1798999

Similar thing happened to me in Ubuntu 18.04. Today is the first day of my installation of 18.04. At first Firefox worked fine. Then I was tweaking a few things after the system upgrade to 18.04, and added the date to the top menu bar / task bar by typing "sudo gsettings set org.gnome.desktop.interface clock-show-date true" in the terminal window at the bash prompt. It didn't show the date, so I rebooted, and then it showed the date. Then I clicked on Firefox. And it was not a black screen, not a white screen, but a clear screen. That is, a transparent screen.

"sudo apparmor_parser -R /etc/apparmor.d/usr.bin.firefox" fixed it for me.

Thank you Olivier Tilloy (osomon) !

I have edited /etc/apparmor.d/usr.bin.firefox in the past when Firefox gave a black screen, after updating to Firefox version 51.0.1 two years ago. What fixed it the black screen then is described here:

  https://bugzilla.mozilla.org/show_bug.cgi?id=1334748#c7

but now my copy of usr.bin.firefox is the same as usr.bin.firefox.dpkg-dist

which I think means it's the same as the current distribution file, although I'm not sure, so I don't know what's going on or why this happened.