It moves the tabs to the left side of the browser and arranges them in an explorer-like tree-view which makes it easy to see which tabs are children of another tab.

I combine it with the Colorful Tabs add-on to make them stand out better.
ANYWAY this is my latest list of Firefox addons if anyone cares. I only use light-weight addons with low resource usage:
- Colorful Tabs
- Flashblock
- IE Tab
- New Tab Homepage
- NoScript
- ShrinkThisLink.com Link Shrinker
- SSL Blacklist
- Tree Style Tab
