I like using Sidebery, which on top of feeling lighter-weight offers a bunch of themes, including a very clean one.

I use the Proton theme with this custom CSS:

https://gist.github.com/wlonkly/6c21134adbdeb6eb257f52076bc3...

and it gives me a nice and minimalist UI consisting of just-dense-enough tab titles, with the current tab highlighted. Looks like so:

https://i.imgur.com/FB59WfX.png

People hate the UI. It was clean before the transition to webextensions, now it's messy and can only be fixed through futzing with userChrome.css (with no help from mozilla.)

If you like my garish and ugly theme, it is TST friendly (it will theme the TST tabs as long as you pick the "Photon" TST theme in the TST settings): https://alan.norbauer.com/projects/alanglow

It looks nice enough to me on both Linux and Windows. If you're on macOS, though, then yeah, I feel your pain.