Fabulous Tabs
That’s what it featured in their “what’s new” page of firefox 5, so don’t forget to have a look to those features:
- App Tabs: Make a permanent home for the sites you visit most. Learn more.
- Switch to Tab: Find already open tabs using the Awesome Bar. Learn more.
- Panorama: Organize your open tabs into groups. Learn more.
http://www.mozilla.com/en-US/firefox/5.0/whatsnew/