Mozilla has released an update to Firefox 3.6 on March 4th. This release addressed an issue where some Java applets would fail to load in Firefox 3.6.14
See the release notes for more details. User should soon be prompted to upgrade but can also can by going to the Help the Menu and selecting Check For Updates…
There is no update for Firefox 3.5, 3.5.17 is the most update version released on March 1st.