As noted, there are quite a few of these, I use this project: https://github.com/Aris-t2/CustomCSSforFx https://github.com/Aris-t2/CustomCSSforFx. It's active

As noted, there are quite a few of these, I use this project: https://github.com/Aris-t2/CustomCSSforFx https://github.com/Aris-t2/CustomCSSforFx. It's actively maintained and lets me put the tab bar directly above the content but you can set it up to fully return you to the days of Firefox 4 if you want...