[pr0zac]

You can still mod Firefox's UI pretty easily, its all defined by CSS files and not too difficult to customize. I for instance actually did the opposite of what you wanted and completely got rid of the tab bar cause I use Sidebery for tab management: https://i.imgur.com/cHz1clI.png Theres a whole subreddit for it even: https://www.reddit.com/r/FirefoxCSS/

This repo (and that subreddit) has all the info on setting stuff up in Firefox: https://github.com/aris-t2/customcssforfx and even has a CSS example for doing multi-tab lines: https://github.com/Aris-t2/CustomCSSforFx/blob/master/curren...

[FeepingCreature]

Sure, but every time I try this it has terrible UX compared to TMP, particularly around scrolling and dragging.

(Also Firefox breaks it once a year or so.)

Not sure how much this is lack of developer effort vs lack of support from Firefox.