I had to prep a new laptop recently, and wanted to put Firefox on it. I'm not sure why; I've prettey much abandoned it for Chrome, but it still seemed like a good idea.
I downloaded the latest version (3.613) from the Mozilla website, installed it, and tried to launch the application. It wouldn't launch. Instead it just crashed on startup. I tried it several times and had no luck. I did get it to launch in safe mode, but that's no fun.
I tried rebooting, uninstalling, reinstalling, redownloading, and kept getting the same result.
Finally, I uninstalled it one more time, then I sought out Firefox 3.5. I found it and installed that version. Once I launched it, it prompted me to upgrade to 3.6. I chose that option, installed the upgrade, and it finally worked.
Short version of the story? Download the earlier version of Firefox, install it, upgrade to the newer version, and have fun surfing.