It's a bug in Firefox. If you know you've installed QuickTime 7.6.6 and Firefox still thinks it's using plug-in version 7.6.3, Firefox is wrong. You can force it to display the correct version number by doing the following steps:
1) Quit Firefox and all other open browsers.
2) Move the QuickTime plugin from "/Library/Internet Plug-Ins/QuickTime Plugin.plugin" to someplace else temporarily (e.g. the desktop).
3) Open Firefox. From the Tools menu, select Add-ons and click on the Plugins tab to verify that QuickTime doesn't show up in the list anymore. Then quit Firefox.
4) Drag the QuickTime plugin back into "/Library/Internet Plug-Ins/" and then open Firefox again. Now when you verify the QuickTime plugin version in Firefox it should display the correct version number.
I came up with this idea and tested it myself, and I can verify that it fixed the problem on my system. These steps should work on others' Macs as well.
