It's a bit technical, and you need to know what you're doing, but here's the nitty-gritty.
There's a "handlers.json" file located in your Firtefox profile folder.
The path is usually %appdata%\Mozilla\Firefox\Profiles\YOUR_PROFILE_NAME .
1. Close Firefox, if open.
2. Create a backup copy of the original untouched "handlers.json" file. Always a good practice.
3. Open the "handlers.json" file in a text editor, like Notepad or Notepad++.
4. Add your custom file types to the "mimeTypes" section.
In this example, I'm adding support for common RAW files of digital cameras, as I want them to get downloaded automatically without Firefox prompting for each one.
You can modify the file extensions and the Content type name to whatever you need (I'm using the name "raw").
You can also choose a different action (0-4), and whether Firefox should ask/prompt you what to do (true/false).
"raw":{"action":0,"ask":false,"extensions":["3fr","arw","cr2","cr3","dng","nef","raf"]}
*Be sure to add a comma where appropriate, before/after the above string!
5. Add the following to the "schemes" section (use the same Content Type name you used in the mimeTypes section):
"raw":{"action":0}
*Again, be sure to add a comma where appropriate, before/after the above string!
6. Save the file.
7. Open Firefox > Settings > General > Scroll down to the Applications table, and confirm that the new entry has been added. By the way, you can modify the action from here - the "handlers.json" file will get updated accordingly.
8. See if you can now download the file types you want without getting prompted for each one.
You should see a download indicator to the right of the URL bar.