Other security fixes include;
-> Vulnerability can be used to gather sensitive data from sites in other windows or inject data or code into those sites, requiring no more than normal browsing actions.
-> Vulnerabilities that would otherwise be High or Critical except they only work in uncommon non-default configurations or require the user to perform complicated and/or unlikely steps.
-> Minor security vulnerabilities such as Denial of Service attacks, minor data leaks, or spoofs. (Undetectable spoofs of SSL indicia would have “High” impact because those are generally used to steal sensitive data intended for other sites.)
Firefox 3.5.1 also fixes stability issues, and an issue that made Firefox 3.5 take a long time to load on some Windows systems.
To get Firefox 3.5.1, first download and install Firefox 3.5, then you can manually update by selecting “Check for Updates…” from the Help menu.