Just because it is not doing anything at the moment doesn't mean it doesn't serve an important, necessary purpose. For a windowed application environment to be functional and usable, there needs to be an easy way to drag windows around the screen. When you start moving things into that space, it makes the system harder to use.
A once-simple action which required minimal thought now requires you to parse an arbitrarily populated area of the screen and find a tiny gap within a litany of buttons and controls and carefully drag that part of the window. If you make a slight mistake and click on a tab or button, the unwanted activation of that control (e.g. switching to a new tab) serves to needlessly penalize the user.
This is not just an issue with web browsers now, but seemingly everywhere. It's been a big issue in the macOS Finder for a while now.
At the very least, Firefox still gives me the option to show the native window title bar, which I very much appreciate. It's certainly not the sexiest part of the UI, given the native element clashes a bit with FF's controls, but at least it's usable! This is an issue that could be solved by giving people a choice via a simple toggle... Most often, the option isn't there.
I'm sorry people have downvoted my post here a bit, and I agree it was a bit strongly worded, but I won't apologize for venting some frustration at what I see as the perpetuation of user-hostile design choices like this.