When Firefox is maximized, tabs start stacking up in the top-left-most corner of the window. When Firefox is open in a smaller window, though, Firefox leaves a blank area to the left of the first tab. I would like the option to remove this blank space.
I've heard the justification that it gives you a place to grab the window, but there is already a blank space to the right of the "all tabs" arrow, so even if the top bar is full of tabs you will always have somewhere to grab. Even in the rare situation that someone is trying to click and drag a background window and can only see its very left margin, there are still other trivial options for moving the window. For example, on Linux you can just hold ALT and you can drag the window no matter where you click on the top bar. You can also just ALT + TAB or click any exposed part of the window to maximize the window you're trying to resize.
The downside of leaving the blank space in the top left corner is that it gives your tabs less space in which to tile, which means there's less readable text on each tab and you have to resort to opening the carousel / "all tabs" list earlier. Given that this occurs in windowed mode, where you already likely have less horizontal space to work with, this is especially problematic.When this was first introduced I honestly thought it was a bug and I haven't warmed on it at all since then. I think it makes the interface look disorganized.