Is the problem that it won't let you change that one from Always Ask to Use Quicktime, or that regardless of what you choose, you end up with the download dialog?

In the dialog, does Firefox show the exact content type? That would need to match up with what is shown in parentheses on the left side of that tab.

On Windows Vista, by default, Firefox will try to play MP3s using the native HTML5 audio player. This relies on the Media Foundation components, so some users have disabled this in order to return to their preferred MP3 plugin or to make it easier to download MP3s. Do you recall ever changing that setting?

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste media and pause while the list is filtered

(3) If media.windows-media-foundation.enabled is in bold type and "user set" to false, you could try double-clicking to switch its value back to true. But this will bypass QuickTime.

The problem is that I want it to play mp3s (via clicking a direct .mp3 link) in-browser via the Quicktime plugin. For some reason, mp3 files aren't opening in-browser like I'm used to from my old computer, but instead can only be downloaded. Upon further investigation, MIDI files are also not working (as in clicking a .MIDI link opens the download dialogue instead of the Quicktime plugin, despite my Applications tab having selected the latter, which is just barely visible in the screenshot), so I suspect a problem with the Quicktime plugin, or that there's some option in the main Quicktime program that I need to enable that I have been unable to find.

The window only identifies the file as "MP3 Format Sound" or "MIDI Sequence".

I have tried it with both the Windows Media Foundation on and off, with the exact same result. (Not that the blasted thing ever worked to begin with - I never encountered the behavior you're describing, which is why I tried the Quicktime plugin.)

Try to rename (or delete) the mimeTypes.rdf file in the Firefox profile folder to reset all file actions.

• http://kb.mozillazine.org/mimeTypes.rdf
• http://kb.mozillazine.org/File_types_and_download_actions#Resetting_download_actions