I failed to reproduce your bug on current release verion of Firefox (72) and Firefox Beta (73). full-screen-api.transition-duration.leave was set to 0 0 and I tried to trigger it by fullscreening random videos on Youtube.
> All other settings were left at 0
Do you mean all the other full-screen-api prefs are set to 0 in you profile? Could you report the exact options that are set when this problem occurs? How does your browser behave if only full-screen-api.transition-duration.leave is 0 0 and all the other settings are at their factory default?
Bug 1609910 Comment 1 Edit History
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I failed to reproduce your bug on the current release version of Firefox (72) and Firefox Beta (73). full-screen-api.transition-duration.leave was set to 0 0 and I tried to trigger it by fullscreening random videos on Youtube.
> All other settings were left at 0
Do you mean all the other full-screen-api prefs are set to 0 in you profile? Could you report the exact options that are set when this problem occurs? How does your browser behave if only full-screen-api.transition-duration.leave is 0 0 and all the other settings are at their factory default?