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I showed the other two since they're popular, and what others would be comparing against. Firefox (on my machines) is more compact than them. So it's not like Firefox is especially wasteful here. One has worse floating tabs, and the other has worse non-floating tabs. So it could be way worse.
Removing all the space would make it super cramped, and I don't think it's worth it for 10-20px. On a typical 1080p screen, that's like 1-2% of the vertical resolution.
That said, it should be configurable. You can probably get what you want with the userChrome.css or whatever it's called.
If they go back to non-floating tabs, you'd save like 2-3px per my screenshots. You seem to want more than that, and that's where the accessibility issues come up.