Affected versions

• Firefox 88.0a1

Affected platforms

• Only macOS 10.15.7

Steps to reproduce

1. Launch Firefox
2. Access customize mode and drag for example "Save page" to the toolbar
3. Enter fullscreen and click on "Save page"

Expected result

• Firefox is not locked out and still can be used

Actual result

• The "Save page" prompt is shown somewhere in background and the browser is locked out

Regression range

• Will see for a regression

Additional notes

• The issue can be seen in the following attachment
• The "Save page" prompt can be forced in if the user is entering the animation of macOS for changing between virtual desktops
• macOS 11.2.3 is not affected.
• Seems like 87.0 or 87 betas are not affected, but older nightlies are affected by this issue (reproducible on 2020-11-16 so far).
• It happens on clean profiles too, with macos-native-full-screen set to false.

This doesn't seem to be a regression. It reproduces way back to Firefox 52.0a1 (2016-09-19) and way beyond that, the builds won't even launch on macOS 10.15.7.

If you go further back the builds will start launching again, there's about a year or so around 2016 where the builds won't launch.

Indeed, I was able to launch 31.0a1 (2014-03-20) and the issue still reproduces, and a 22.0a1 (2013-03-20) but there is no longer a "save page" option to drag on the toolbar. Thank you!

I think this should be a duplicate of Bug 1638866 because the different virtual desktop is the root issue. (maximized windows open on a different virtual desktop in macos)
https://bugzilla.mozilla.org/show_bug.cgi?id=1638866