I assume you added that red line for illustration, and you don't actually have a bright red border/title bar on your Firefox.

In a maximized window, there should not be a gap above the tabs. Some users who have multiple displays with different resolutions sometimes have a 1-pixel gap above the tabs on one of their displays; possibly a rounding error when Firefox is layout out the screen. Yours looks huge, however.

Have you checked whether the same problem occurs with your userChrome.css rules disabled?

Edit: I should not assume you set up a userChrome.css file, but otherwise it is difficult to imagine how this problem could occur.

Initially, I did not create userChrome.css and tried to solve this problem by resetting, reinstalling, and then I created it, but as if I didn’t change it, it had no effect and didn’t help respectively. The strangest thing is that this problem only on my computer account, the second account with the same settings, everything works correctly

After the last update for FF for some reason, the compatibility mode setting has been enabled. It turns out she was the cause of this trouble.

Thanks for the update. Do you mean running Firefox as though it was on an older version of Windows instead of Windows 10, or something specific to display settings? I guess either way, turning off compatibility settings solved the problem.