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AFAIK, Tree Style Tab still exists? https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/ https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta

• rkangel 2y ago
  It does, but you have to have some custom CSS to turn off the normal horizontal tabs, and you have to go and enable some options in about:config to even have custom CSS. I've got something that works reasonably well, but it's really hard to have CSS that always works correctly.
• Izkata 2y ago
  The original XUL-based vertical tabs actually moved and restyled Firefox's native tabs, instead of creating a lookalike (which is all that's possible now). This meant that unrelated addons that did things like grey-out unloaded tabs, or alter the favicons to have some sort of indicator, still worked on the vertical tabs. The current vertical tabs addons all have to do it themselves or add some sort of API for the other addons to interact with to get the same effect.