Rated 5 out of 5 stars
In the world of widescreens, vertical tabs is the only thing that makes sense. I'm a power user and constantly find myself multi-tasking and forking my work off into tangents - the tree structure and coloured highlighting have been revolutionary and removed the need for me to ever have to open more than a single firefox window. I've been using this add-on for several years now and it's great - thanks! Be sure to grab yourself tab highlighter - a must so that you'll have any clue, which tab out of the 20+ you have open you're currently looking at!This review is for a previous version of the add-on (0.10.1.1-signed).
The original author John Mellor (Jomel) did most of the work I suppose :P
I do love vertical tab bar more than horizontal one too since the original Tab Kit. It is the biggest reason (plus other extensions) for me to stay with Firefox.
Chrome had vertical tab bar as experimental feature once but removed soon after, failed.