I can play some quicktime files when they are embedded in firefox, such as mp3's or .mpgs, but usually h264's or basically most video codecs don't work and I just get this error, and firefox crashes.
I used to get something similar sometimes. Here is a weird little thing that could be it.
If you have more than one user on your OS, quicktime only works properly with the user that you installed quicktime under. It won't work right with the other.
Yay! I figured out the problem. I was doing some research and found that there's some problem between my Nvidia drivers and the quicktime software... so it's not really firefox. If anyone else is having this problem what you can do is go into the quicktime preferences > advanced > and under video either click on safe mode (gdi) or just uncheck the two direct draw boxes. Something with direct draw messes up the plugin for firefox / ie. But now it works