The latest & final version of firefox 3 was released by the Mozilla Foundation on June 17,2008.
It was named "Download Day 2008". Firefox was aiming to set the record for most software downloads in 24 hours. Although there is no current standing record for this feat.
8,249,092 unique downloads were recorded when Download Day ended at 11:16 am PDT June 18 setting the Guiness World Record for maximum software downloads in a day.
Within 24 hours Firefox 3 usage rose from 1% to 3%.

Now coming to the features which makes Firefox unique from other browsers:
1. Tabbed Browsing
2. Pop-up Blocking
3. Integrated Customisable Download Manager
4. Live Bookmarks
5. Cross Platform Support
6. Add-Ons Support

For a detailed list of features please visit the link below:
http://en.wikipedia.org/wiki/Firefox