I filed it at bugzilla:
https://bugzilla.mozilla.org/show_bug.cgi?id=1571173
** Bug watch added: Mozilla Bugzilla #1571173
https://bugzilla.mozilla.org/show_bug.cgi?id=1571173
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Public bug reported:
I took a video. It also seems to show a minor bug with the built-in
video capturing program accessed with ctrl-alt-shift-r
https://youtu.be/1gHPMakVzSA
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: firefox 68.0.1+build1-0ubuntu0.18.04.1
ProcVersionSignature: Ubuntu 4