RE: Horizontal tabs are now slightly more rounded to match the look of vertical tabs. Buttons and text inputs, including the address bar, have also been updated for consistency. (Firefox 145.0 changelog)
This change introduced a bunch of dead space at the top of the window when the window is maximized. That means you need a lot more precision when you are trying to select the tab. I used to be able to flick the mouse to the edge of the screen and pick the tab quickly because I didn't have to worry about aiming the cursor vertically. But now that I can't click on the edge of the screen, the vertical space where my mouse cursor is allowed to be to do what I want goes from infinity to just a few pixels.
It's a huge downgrade from UX and accessibility perspectives. I would suggest removing the dead space at the top of the window and make sure users can click in the title bar. When I go in full screen mode (F11), the dead pixels aren't there and the app works perfectly fine, so the problem is with the maximized window.
Check out Fitts's Law for the importance of infinite edges in UI design.
