User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:84.0) Gecko/20100101 Firefox/84.0
Steps to reproduce:
I set the titlebar buttons to the left.
The titlebar buttons have been set correctly, but there is an empty space.
The space shouldn't be there.
I was able to replicate this on Ubuntu 20 64bit installing the tweak application.
There seems to be a big gap between the buttons and the tabs when enabling right to left titlebar buttons.
It also happens on:
Firefox release 83,
Firefox Beta 84.0b8
Firefox nightly 85.0a1
This space on the left also appears on XFCE when the buttons are on the right. The gap goes away when the window is maximized.