The new version addresses several security issues as well as stability issues.
(Version 3.5.3 release notes are posted here).
Most users will be automatically updated but if you aren't yet seeing the latest version, you can simple click to 'Help' button at the top of the browser and then, 'Check for Updates'. By the time you read this, you should be seeing 'downloading version 3.5.3'.
The complete list of language and platform specific versions of Firefox 3.5.3 can be found here.
Note: While Mozilla is continuing to urge users of their previous generation web browser 3.0 to move to the 3.5 series, an update for the older browser has been released bringing that version to 3.0.14.