From the Beta Download site, you can download Firefox 3.5 Beta 4. But from there, pressing the Check for Updates button would update your Firefox to a version known as 3.5b99, which is a preview version, between beta and RC.
Just today, another update has been pushed out. This one happens to be the Release Candidate.
"Because of the 800,000 or so testers that Mozilla says have been using the beta versions, Firefox director Mike Beltzner said that he expects this to be the sole release candidate before version 3.5 goes public at the end of June"
That is certainly great news. More than one year after Firefox 3.0, the what was supposed to be a minor update 3.1 released in December is now 3.5 and contains many new features.
There aren't really any noticable changes between the beta/preview and the RC. Just expect it to be more stable.
Here is an excerpt from CNET:
More improvements include support for HTML5 tags such as < video > and < audio >, native JSON support, support for Web workers so browser-based apps can run in the background, support for CSS and SVG standards, the ability to erase browsing traces by site or by time, personas for easier theme management, and downloadable fonts. The release candidate is also available in more than 70 language localizations."
Note: In Firefox 3.5 RC, it doesn't seem to actually say RC anywhere except the welcome page. The About window just shows 3.5.
Update 6/19: Firefox 3.5 RC 2 is now available for download here. Fixes a few bugs and stability issues found in RC 1.