Firefox released 3.6 today with a few notable improvements.

Changes were made that prevent other programs from adding their own toolbar to Firefox without your permission.
Firefox 3.6 will alert you about out of date and insecure plugins.

Private browsing also removes TEMP files.

The full details can be found at http://support.mozilla.com/en-US/kb/Upgrading+to+Firefox+3.6 and the upgrade can be downloaded from http://www.mozilla.com/en-US/firefox/upgrade.html