They unfortunately ruined TST with the switch to WebExtensions because it could no longer replace the top tab bar. There were hacks you could do by modifying something in your Firefox profile directory.

Bizarre to me that they didn't take TST's popularity as a hint of supporting vertical tabs as a native feature until almost 7 years later.

I want to say they actually put some work in to allow TST to still work when they transitioned to quantum as it was a popular extension.

I got the impression they were eventually going to add back in the horizontal tab bar disable as well but if that was even true they clearly forgot about it. I've been using the userChrome hacks for close to 7 years now.

Compared to pushing and implementing CSS, JS and HTML standards, cryptography, APIs, mobile OS release cycles etc, I've always wondered whether vertical tabs would've been an easy win.

Yeah, I remember it being a huge internal argument against WebExtensions at the time. (But, security + easy of building + compatibility + speed of developing Firefox + a bunch of other things made the switch off XUL the right choice)