https://www.reddit.com/r/firefox/comments/buwjzp/firefox_bugging_out_tabs/ might have the same issue.
Bug 1542668 Comment 4 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
https://www.reddit.com/r/firefox/comments/buwjzp/firefox_bugging_out_tabs/eplhkf2/ Task bar at the top, unusual drag space visible when window is maximized: https://i.imgur.com/sbHYpiy.png
https://www.reddit.com/r/firefox/comments/buwjzp/firefox_bugging_out_tabs/eplhkf2/ Task bar at the top, unusual drag space visible when Firefox is maximized: https://i.imgur.com/sbHYpiy.png
https://www.reddit.com/r/firefox/comments/buwjzp/firefox_bugging_out_tabs/eplhkf2/ Task bar at the top, unexpected drag space visible when Firefox is maximized: https://i.imgur.com/sbHYpiy.png