Windows/Mac/Linux: The release candidate is out for Firefox 3.6.4, an update that will bring much-needed plug-in isolation and crash-proofing to Firefox's stable build. If you want to check it out early, the all-but-finished release candidate is yours for the taking.
Among the many bug fixes made to bring Firefox 3.6.4 up to near-release condition, many have centred on Flash and Silverlight operation, so those using a beta of 3.6.4 have little to lose in upgrading. Everyone else can get a taste of the plug-in isolation that keeps your entire Firefox session from going down, just because a Flash video is having some problems.
Firefox 3.6.4 release candidate (labelled "beta build 6", but confirmed as an RC) is a free download, available for all platforms.