Brilliant. Had been using Tree Style Tab for the last few years but that took a lot of configuring (and re-configuring) and I recently noticed it was significantly impacting start-up time. Had been waiting (for ages!) for vertical tabs to be supported natively, but was very disappointed with Firefox 136.

Decided to give Sidebery a go and I'm impressed. Minimal customisation required to get it looking/behaving how I want, and it has significantly speeded things up.

Thank you!