One of the best add-ons you can get for Firefox! Many customization options, easy to use, fast, reliable, nice UI, good support ... I could go on.
I use it everyday at work to organize my tabs. I am editing pages in Confluence in one window, and open tabs with background information (that I stored in a dedicated tab group) in a separate window in just one mouse-click! I also have tab groups for my different projects.
Highly recommended and many thanks to the developer!