Firefox is trying to be flexible. To that end it learns from experience. It connects extension / mimetype / handler information in a user profile file called mimeTypes.rdf, in the folder \AppData\Roaming\Mozilla\Firefox\Profiles\*******.default of your user folder.

It is an RDF file that connects mimetypes and file extensions to handlers and handlers to external applications. And whenever you download a file with a new mimetype, this mimetype is stored along with the extension of the file. If you chose to open the file with a given application and do so automatically in the future, this information will be stored as well.

Web-applications can send any mimetype they like, along with a file download. Most applications will send the correct mimetype or just omit it. But some applications send the incorrect or non-existing mimetypes mentioned above. And Firefox has no way of telling these apart as bogus.

Fortunately, Firefox doesn't just use mimeTypes.rdf, it first looks in a hardcoded override list, then in mimeTypes.rdf, then asks the OS [source].

Now, it also works the other way around: if you upload a file from your own filesystem to a website, Firefox checks mimeTypes.rdf to see which mimetype it should send. Since a mimetype may be connected with several extensions in mimeTypes.rdf, Firefox needs to choose. Some quick testing leads me to believe that it takes the first occurrance.

This problem is acknowledged by Firefox, mainly because it corrupts its internal handling of SVG files. [source]