I am running Mac OS 10.6.8 and my Quicktime plugin (7.6.6) works completely in Safari, with help in Chrome and not at all in FireFox, even with all other plugins disabled.
Firefox will play QT movies from the Apple Trailers site and up until maybe two months ago, played QT movies and MP3 files from my personal site. I can't hear the MP3 file that auto-loads at:
With other browsers:
Chrome asks for permission to use the QT plugin for each site.
Safari works fine.
I uninstalled/re-installed FireFox and found that the QT plugin worked the first time it came to a page with recognizable media but never after that. Not after reloading or even after shutting down the computer and re-opening FireFox.