I've encountered the following problem on a system running Firefox on Windows XP: when you go to a page that has Quicktime movie(s) embedded (not sure if that's the word; I mean, that you can play directly on the page, as opposed to clicking on a link to open the file), the following error message comes up:
QuickTime failed to initialize. Error # 0
Please make sure QuickTime is properly installed on this computer.
That's in one dialog box. After clicking OK, another dialog box comes up, with the message
The plug-in performed an illegal operation. You are strongly advised to restart Firefox.
and a checkbox to "not display this message again in the future", or something like that. (sorry, I'm not in front of the system right now)
This second dialog box apparently comes up once for each quicktime object in the page (or so it seems; I haven't counted them exactly, but in one page with several QT movies it came up several times, in another one with just one it came up only once)
Before this error came up, someone had installed iTunes on the system, but the error didn't start appearing immediately. Apparently the computer was running for about 3 days without any problems, although I suspect in that time the system wasn't rebooted, and after it was rebooted it started acting up. Before iTunes, QuickTime always worked correctly.
Before I started trying to fix this, the Firefox version was 1.0.x, I believe it was 1.0.7. In the process of trying to fix it I upgraded it to the latest version (1.5.2?)
QuickTime plays fine when running stand-alone, after clicking on a local MOV file; it also runs ok from inside IE (tested with the same sites I was trying Firefox on).
I tried, over and over and in different sequences, to uninstall and reinstall Firefox, QuickTime and iTunes. I tried erasing the directories for Firefox (c:\program files\mozilla firefox) and QuickTime (c:\program files\quicktime), as well as deleting some registry entries related to them, after uninstalling them, to try to get a clean installation. Nothing seemed to make any difference.