On macos when you go into customize mode and check "Title Bar" at the bottom it adds some spacing to the left side of the tabs bar to avoid the tabs overlapping with the mac titlebar buttons (close, minimize, maximize buttons).

However, I have used userChrome.css in order to move the tab bar to the bottom. At first the navigation buttons were overlapping with the titlebar buttons but that was easily fixed by just adding a "Flexible Space" element on the left. But no matter what I do I cannot get that spacing on the left of the tabs to go away unless I uncheck "Title Bar", but I would prefer to not have to do that.