Those screenshots look really nice, ngl, hoping this goes through. Edge and Vivaldi have had their own vertical tab implementations for a while now, and there are Firefox forks that show it can be done. No reason for base Firefox not to have it at this point.
While I'm here, Mozilla bring back compact spacing, plz k thx.
Edit: Just tried it, it's got that nightly jank but it's promising. I hope Mozilla continues with this. It looks and feels great.