One possibility is that the server is sending a novel Content-Type with the download. But if it's a multi-site problem, it's probably a stored data problem: Firefox stores file handling actions as you browse, and it may have stored the .mpga extension from one download as the standard file extension for "audio/mpeg" files.

Download actions are stored in the handlers.json file in your profile. The JSON format is difficult to edit/clean, but as a blunt approach, you can rename the file to hide it from Firefox and Firefox will start a new one from scratch.

To try that:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, find the Profile Folder row. Click the "Open Folder" or "Open Directory" button on that row to launch the folder in your system file browser.

Leaving that window open, switch back to Firefox and Exit/Quit, either:

• "3-bar" menu button > Exit/Quit
• (menu bar) File > Exit/Quit

Pause while Firefox finishes its cleanup, then:

Right-click handlers.json then Rename to something like handlersOLD.json

Start Firefox back up and try the download again.

If you lose too many useful actions in this process, you could replace the new handlers.json with the old file.

You can possibly check the HTTP response headers for this request in the Network Monitor.

• https://developer.mozilla.org/en-US/docs/Tools/Network_Monitor

https://bugzilla.mozilla.org/show_bug.cgi?id=1753090