I'm late to updating to the latest build of FF from the "old days" - mainly because I was very, VERY attached to some of me add-ons. The most cherished of these revolved around how I managed my tabbed experience. I tend to have a LOT of open tabs & tab trees; and as such, I've been using a vertical, "tree-stye", and colorful tab experience. The three "legacy" add-ons I primarily used were Tree-Style Tabs, Colorful Tabs, and Tab Mix Plus. I waited until upgraded versions or suitable replacements existed for these.
Turns out, Tab Center Redux is one of these, and one of the better ones at that.
I am very impressed with the assortment of features available in this add-on, and especially how they are integrated. For instance, I really like having the "New Tab", "Search Tab", and "Options" available in a single bar at the top of the tab sidebar - the less clicks the better and all that.
One of the most clever features I've found is the ability to truly customize the look-n-feel of the tab sidebar via the use of CSS/HTML. Since most geeks and power users (definitely me, and probably you too if you're reading this ;) ) are notoriously picky about how their toys look, this allows us to customize to our heart's content (and it releases the original developer of this tool from the unenviable task of trying to satisfy this crowd, too - well played. :D)
I didn't give Tab Center Redux five stars because a) I haven't had a chance to really dig in and see exactly what it can do, and b) I don't want to reward or punish the developer for the sins or virtues of Firefox itself. If it's out of the dev's hands, then it's not their fault. After I become more familiar with the tool I'll happily raise (or lower) my rating accordingly.
Simply put, Tab Center Redux is definitely worth an install and look-see if you're in the market for a vertical tab bar with some nice features and customization possibilities. Most likely you'll be pleasantly surprised - and you could definitely could do a lot worse.