Is there a limited set of filetypes that firefox will choose to save to browsing cache or can any filetype theoretically be cached? obviously images, scripts, and other media make sense but could a website also force the browser to save things like zip files or exes?

1 Answer 1


For MIME types listed in /handlers.json, you can choose whether certain file types are to be saved, to be opened by a specific application, or handled some other way. Though some MIME types are already listed in about:preferences, you can add more types by editing handlers.json.

Firefox MIME handler

This allows you to decide how file types are processed. In the screenshot, note that I've changed handling of EPUB file, so that all those goodies on Project Gutenberg or the Internet Archive can be saved to the default location with a single click.

AFAIK, there is no way to store an active executable file in the caches, e.g. in Windows 10:

  • C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile\cache2\entries
  • C:\Users\<user>\AppData\LocalLow\Mozilla\tempxxxxx

That said, it might be possible to store such inside another file, such as an image, through steganography.

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