User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

1. Start Firefox

Actual results:

Firefox opens maximized, then after 1-2 seconds max. it "restores down" its window. Clicking on the page, links (a href) inside the tab doesn't work. Clicking on the "X" button of the shown half-way visible error message from Ubuntu doesn't work as it does not focus and send mouse click signals to the Ubuntu problem report window, because Firefox somehow IMO uses this whole transparent area to the full right edge of the screen, but doesn't paint itself/its window there. (Please note that the mentioned Ubuntu bug reporter window has nothing to do with Firefox or reproducing this bug.)

I've switched recently to Wayland, but this issue was reproducing very frequently even when i was using X.org, and it was reproducing for months (i actually don't remember since when). I don't know why i report the bug only now.

How i fix this issue is, since the minimize/maximize buttons of the Firefox window don't work when the issue happens, i use Super + Down arrow, which usually when the window is maximised restores down the window, but when the bug happens, when i press Super + Down arrow for a brief moment shows the window maximised, and then immediately restores down the window to what it was before. The only difference between the buggy "restored down" and the "restore down" that happens when i press Super + Down is that now it works properly, doesn't cover with transparency the windows near it (if existing), and clicking on pages and the maximise button work.

Expected results:

Firefox should have opened and stayed in maximized mode, should not have automatically entered this buggy "restored down" window mode.

This reproduces not only in Nightly, but also when i was using the Ubuntu repository's default Firefox.

I'm sorry, but the screenshot i've taken with the buggy "restored down" window was lost in my clipboard. I'll take a new one when the issue happens again.

The issue usually happens in cold boot, and during the initial loading of the page the whole Firefox window and the notification from Ubuntu which was showing at the same time were flickering as in - getting to a darker shade of grey, then becoming okay, then again darker shade of grey, then again becoming okay, and this was changing maybe 10 times in a second. I guess the issue with the Ubuntu notification flickering the same as the Firefox window was because somehow Firefox was covering it with its transparency.

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

The attached video shows Firefox after i launched it and it automatically "restored down" its window to a somehow broken state. Although it doesn't display itself all the way to the top, when i clicked over "lyubo" in Nautilus file bar, Firefox catched the click instead and showed it's URL suggestions instead of Nautilus changing the opened folders. I clicked once more as viewable in the video and Firefox changed the tabs.