Whenever I see someone using (or a screenshot of) horizontal tabs I'm just shocked that people put up with it, or perhaps even more so that it's still the default in all major (and minor?) browsers. I recommend Sidebery, but more than that I just recommend something to get your tabs listed vertically in a sidebar, so they don't squash each other up as you add more until you can't tell what's what at a glance, the wid…
Most users, like 90% or more have tabs that all fit visibly in the horizontal space. Look it up. Mozilla's telemetry is public. 5 or 6 tabs fit just fine and in a sidebar are hugely wasteful of pixels. You think everyone browses just like you so you can't imagine why the browser is shaped for people not like you but your browsing habits are in the ultra-minority. Take a look at the telemetry before making silly posts…
You know what is also a waste of pixels? Websites pretending they are mobile view only on my 4K desktop monitor in landscape. Blogs, new Reddit, Twitter, Mastodon, news sites, whatever else.
The vertical tabs wouldn't hurt that shit at all.
And if the vertical tabs would be a first class citizen in a browser it would greatly help so if needed I could have switch to it.