More info:

This is on another machine, so you can ignore the "more system details" info.

All other plugins appear in the list, and from what I can tell, work just fine. Not Quicktime.

The plugin does not appear in the Plugins list on freshly generated user accounts on Mac OS 10.6.8. I've tried both admin accounts and standard accounts. Nor does it show up under about:plugins.

Tested with Firefox 3, 5 and 6. Tested with a clean install of Firefox 6 (removed system/user support files).

I've tested a copy of the 7.6.6 plugin copied from another machine, as well as an old 7.6.3 copy.

The file ownership/permissions seem normal on the /Library/Internet Plug-Ins/ folder as well as the plugin package itself. But just in case I recursively set the permissions as well. No effect.

I've tried reinstalling Quicktime 7 from the OS Disk.

I've tried disabling all plugins, adding the plugin manually, scanning for plugin updates. I've also "re-initialized" the Firefox plugins database by deleting it. Everything covered by your official Troubleshooting Plugins wiki.

My theory is that Firefox is looking for something that Mac OS is not providing. The plugin itself is plainly available, but Firefox doesn't seem to realize it. My guess is that Firefox doesn't look at the Internet Plug-ins folder itself, but relies on some function of Mac OS to relay the existence of "internet plugins", or Quicktime in particular. But who knows.

It turns out that the default user profile was not generating a com.apple.quicktime.plugin.preferences.plist file, which contains all the mime types for the browser. Putting this in the default user profile fixed any newly generated user profiles.

For existing profiles the only thing that seemed to work was going here: http://ctarabians.com/testwmv.html and allowing Firefox to change into 32 bit mode. This somehow enabled the plugin to be recognized in lieu of the plist. I think you can also right click on the Firefox.app and checkmark 32 bit mode.