Some features include
- Places - a new bookmarking system (see MozillaWiki:Places)
- Microformats - native support for this may be included in Firefox 3 (see MozillaWiki:Microformats and Microformat handling)
According to some, Firefox 3 is only a more organized version of the latest, 188.8.131.52. Of course, it will still have some bugs, but they will be minor.