Sometimes this is a Firefox settings file problem: the file which stores download actions can't be updated or becomes corrupted with nonsensical values. You can dump the current file and have Firefox build a new one. Steps below.

Sometimes this is a website problem: the server sends a generic Content-Type indicating it is a binary file. Firefox doesn't associate a specific action with that generic indicator. Assuming it isn't your server, a potential workaround would be to use an add-on. Details below.

Triggering a Rebuild of handlers.json

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

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

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in File Explorer.

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

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

Pause while Firefox finishes its cleanup, then rename handlers.json to something like handlersOLD.json. If you see a file named mimeTypes.rdf, you can remove it.

Start Firefox back up again. Any improvement?

Workaround for General or Wrong Content-Type

I created an extension which recognizes common file extensions like .pdf, .xlsx and .zip and assigns "correct" Content-Type headers to them. It also lets you assign content types to more exotic file extensions like .torrent since I didn't think to add that one. If you want to give it a try:

https://addons.mozilla.org/firefox/addon/content-type-fixer/

When you first it install it, the extension is snoozing (Zzzz button). Click the button to turn it on and try a .torrent download. Then click the extension button and choose the lower button, Add/Edit Content Types. Here you should get a list with an option to define a new content type which either can be the "official" content type application/x-bittorrent or a made-up type. After saving that, try your download again and see whether you can now store your preference persistently going forward.

Any improvement?

Since the extension intercepts all page requests and downloads, it can slightly slow down browsing, so I suggest snoozing it until you find a site where you need it.

Note that "Do this automatically for files like this from now on" is about the content type send by the server and not about the file extension. If the server sends the file as a generic content type then Firefox will still show the download dialog.