FireFox 3.5.1 has been released, which patches the zero-day vulnerability found that could allow arbitrary code execution if browsing to a remote site. It also fixes startup problems that were created thanks to scanning of multiple disk directories on startup; having run the Mac version through monitoring the file system, then I can confirm that it does as advertised.
If you use FireFox, and you'd installed the 3.5 update, then you should install this update immediately. If you followed instructions to disable the javascript.options.jit.content
setting, you might want to re-set this back to 'true'.