Firefox uses whatever your computer's default applications.

Usually, during installation, there's a point where you can select file types Quicktime will open. You will sometimes be asked if you'll give permission for a program you're installiong to 'reclaim' file types. If you check that, you've got a problem.

Otherwise, in Windows XP,

go to Windows Explorer
Find a file of the type you want to change
Right click your mouse on it,
Select 'Open with'
Select 'Choose program'
Find the right program, and click on it once to select it
Put a check mark in the box underneath that says 'Always use the selected program to open this type of file'
Click 'OK', and you should be done