> Firefox only recently gained an option to put tabs on the side

regained. And I don't think it was a long break at all. tree organization for those side tabs, now that took a lot of time to regain, after they ripped API used by TreeStyleTabs extension.

If they indeed adopted Tree Style Tab, that is, allowed tabs to form a tree, that would be wonderful. Alas, I sill have to run TST and hide native tabs.