It's not Firefox 3.5 yet, but Mozilla has an update available to the Firefox 3.0x browser today. Firefox 3.0.9 is now available for download here. Current Firefox users should be prompted to upgrade within the next two days.
Firefox 3.0.9 fixes a number of security and stability issues. It also repairs a bug in which a corrupted database would cause stored cookies to disappear, as well as a glitch that prevented embedded images from showing up within e-mails on various Web-based mail services.
Two other issues are corrected in the update: one in which new windows would not focus properly, and another in which large forms would submit too slowly.
As for Firefox 3.5 Beta 4, it should be on the way any day now. The fourth beta was originally set to have its code frozen last week, but that was evidently delayed.