How can I get Firefox to display MIME types in-browser?

Solution 1:

Here's an add-on that might help: "Open in browser"

And here's a KB article that also might help, "MimeTypes"

Solution 2:

In general you should use the "tools->options->applications" to edit which application is used by default to open different mime types. However if you can't set it that way you can edit the mimeTypes.rdf directly, but this is discouraged.

Adding the following should work (use at your own risk):

<RDF:Description RDF:about="urn:mimetype:application/json"
           NC:value="application/json"
           NC:path="<path to firefox>"
           NC:saveToDisk="false"
           NC:editable="true"
           NC:prettyName="JSON object">
<NC:handlerProp RDF:resource="urn:mimetype:handler:application/json"/>
</RDF:Description>

Solution 3:

http://support.mozilla.com/en-US/kb/Managing+file+types

If you open the preferences windows and go to the Applications tab, you should be able to find the content type (application/json) and change its action to "Use Firefox"