The latest version of the Firefox is currently being distributed to all official Mozilla mirror servers around the world. Some of them are already hosting the latest version of Mozilla Firefox 3.5.2 for all supported languages and operating systems while others are only showing the directory structure with no files yet.
Most probably, it is likely going to take at least a few hours more or even more than a day before all servers are distributed with the latest version. Then the release can be officially announced on the Mozilla website. Usually it takes around 24 to 48 hours before the automatic update checker in Firefox shows the new release information. So, if you rely on the Firefox automatic update checker then you will have to wait for that amount of time.
If you are interested in the release notes, you may read the beta release notes of Firefox 3.5.2 mention. The beta release notes states several security and stability issues that have been fixed in the new version. This update is therefore a recommend update for all Firefox 3.5.x users. An issue with images with ICC profiles has been fixed as well which now renders properly on all computer monitors. The full list of bug fixes and issues can be accessed at Bugzilla.