The current #firefox tabs were fine for me tb, but that useless gap in front of them when using windowed mode wasn't.
After messing around with userChrome for 5 mins, I'm much happier with how it looks.
PS: tested and working on both Manjaro and Windows, FF Dev. Edt.
Here's the code that does it: https://pastebin.com/8m6fwGbJ
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