I recently started getting this message in Firefox:
QuickTime failed to initialize. Error # 0
Please make sure QuickTime is properly installed on this computer.
It seems it's caused by launching Firefox in WinXP compatibility mode, which I did in order to be able to use Rhapsody's music service, since they prevent it from running under Win2k3. Yucko. In any case, I searched over the web and nobody else had figured out why Quicktime was crashing this way. Perhaps there are other causes, but this one fixed it for me. I didn't even have to reinstall Quicktime.