This is the single most useful add-in for Firefox, and is actually the major reason that I still use Firefox as my primary browser. The ability to organize tabs into groups so that I can manage tasks, and easily switch between groups related to the various tasks and projects that I am working on makes my life so much easier. If you have more than a few dozen tabs this becomes the _only_ way to effectively manage them. It's so useful that it's used by pretty much every developer in our organization.