@aetios Yeah, although hidden in about:config as sidebar.revamp and sidebar.verticalTabs. They're less janky than Zen Browser's, though (I can't reorder tabs there)
@anthropy Let's hope Zen Browser puts more focus on QA in the future. Whatever they're doing now is a bit annoying, but the software is also labelled as being alpha, now beta
I think I'll switch back to Firefox. Well-integrated vertical tabs were the main reason I switched, but it looks like Firefox' implementation of vertical tabs is fulfilling this now.
Volpeon