> We're cautious about not repeating history. We're implementing tab groups from scratch and directly in the tab bar. Our Firefox View feature (Tools > Firefox View) may later get a more visual surface for managing groups.

Hi! Thanks for letting us know about Firefox Team is being very careful this time, I want to share my idea too because I think the Firefox community have probably tinkered enough to figure out that Tree Styled Tab Group like Sidebery/Tree Style Tab is the best Tab Group implementation, because they're so easy to use, just drag and drop and work great in practice.

I hope you consider making Tree Styled Tab Group an option for vertical tab.

In ideal world, but you should consider about cross-platform capability and usability, most os window managements sucks, that's why web browsers implement their own.