If you habitually have a lot of tabs open, you'll probably know how annoying it is finding things when each page title has been condensed down to 4-5 characters. On widescreen displays (especially 16:9), vertical pixels are also a lot more precious, while horizontal ones are plentiful.
For me (3840×2160 display, 200% scale), its vertical tab sidebar fits about 30 tabs before needing a scrollbar, and you get a full width title for each and every one.
It can be a bit of an adjustment at first, but I've been using this since the pre-WebExtensions days (since around Firefox 4.0), it's definitely one of my must-haves.
This changed my habitual way of working with browsers for the better, can't recommend it enough. I'm using Sidebery though, not sure of the differences, but I really like its snapshot feature.
Not only does it trade off precious vertical space for plentiful horizontal space, but also the tabs get organized hierarchical, so when searching and opening multiple tabs , the tabs get grouped naturally