No matter how hard I tried, besides disabling the Quicktime plugin in Vista, moving/deleting npqtplugin4.dll in the Firefox plugins folder or hex editing it, Firefox 3.0.1 would always play any mp3s I clicked in a full screen Quicktime plugin.

The about:plugins shows the following:

QuickTime Plug-in 7.5 (861)

File name:
npqtplugin4.dll

The QuickTime Plugin allows you to view a wide variety of multimedia content in Web pages. For more information, visit the QuickTime Web site.

How the hell do you change one of those values to say “enabled: no”?

Changing Mime Type associations in Firefox via Options -> Applications does nothing – Quicktime always gets there first

Changing Mime Settings in Quicktime Preferences launches Vista’s Set Program Associations which is also fruitless

After much digging around, I have worked out that the following will let you handle audio/mpeg (mp3) the way you want:

• Go to about:config
• Type ‘plugin’ in the filter box (minus quotes)
• Look for ‘plugin.disable_full_page_plugin_for_types’
• If it already exists and contains a value, add ‘,audio/mpeg’ to the end of it, otherwise just change the value to ‘audio/mpeg’
• Fully close Firefox, wait a few seconds for it to clean up and die
• Launch Firefox again.. click an MP3 link.. voila! it does whatever you’ve specified in your Firefox Options -> Applications