It also doeans't load Macromedia Flash files. I tried uninstalling and re-installing and upgrading. nothing works. Opera & Internet Explorer work fine but firefox is a pain in the A**
Get this: https://addons.mozilla.org/firefox/446/ First, install it and then follow the directions on screen. Once you restart Firefox, all of your Windows Media Player and so forth browser players will be replaced with a image. When you click on it, bada boom, bada bang. It'll open up the media player that corresponds and you should then be problem free. Enjoy.
Try uninstalling Firefox and quicktime (Export your bookmarks if you have anything precious) and then find and delete all the Firefox user profiles. Then reinstall it as well as reinstall QuickTime Alternative, Flash, etc. from scratch.