You know what we should do? We should bring back Galeon. That was a good browser. https://en.wikipedia.org/wiki/Galeon
I see that the new Firefox release has native vertical tabs. Only 20 years late. 
https://bugzilla.mozilla.org/show_bug.cgi?id=218280
I'm having a look at what Epiphany (I'm sorry, "GNOME Web") is like these days; if I can't find an extension to put tabs on the right, this is going to be a very short investigation.
You can enable the native vertical tabs in FF already with about:config. I'm trying them out; I've used Tree Style Tab for a long time, but I don't really use most of the features, and one fewer extension is not a bad thing.
I'm going to have to restyle these pinned tabs, though; they're taking up way too much space.
@confluency I've been using Tab Center Reborn with a bit of custom CSS for ages. Native vertical tabs still feel somewhat clunky to me. Could probably live with them after some restyling, but this still works well so I don't bother 🤷
@dos I'm going to try out the native tabs for a bit. What I really want right now is the ability to inject the old ChromaTabs Plus background colour functionality using the autoconfig / userChrome.js loader hack, which I think could also replace everything I'm currently getting from firefox-omni-tweaks in a cleaner, persistent way. But that's going to be really annoying to put together, so I'm going to have a think about whether this is really what I want to do with my life.