A workaround is to use the view-source: protocol to force Firefox to display the file in a Firefox tab as plain text. You can use a bookmarklet to enforce the builtin JSON viewer.

javascript:location.href='data:application/json;charset=utf-8;base64,'+btoa(document.body.textContent);

Hi cor-el.

Not sure how I'd use a bookmarklet, as firefox can only save the file.

Here's a public URL that generates a response with the Content-type header "application/vnd.api+json".

https://discuss.flarum.org/api/discussions?include=startUser%2ClastUser%2CstartPost%2Ctags

Typing this URL into firefox (or clicking the link) causes a dialog box to appear which gives me the option of saving the file, or opening it with another application (which also saves the file).

By contrast, here is a public URL that uses the "application/json" content type, and which is displayed within firefox.

https://jsonplaceholder.typicode.com/users

Prefix to URL with view-source: to make Firefox display the file in a tab. view-source:https://discuss.flarum.org/api/discussions?include=startUser%2ClastUser%2CstartPost%2Ctags

You can create a new bookmark and paste the JavaScript code in its location field. if you want to test this then run the JavaScript code via the command line of the Web Console.

I have found a plugin https://jsonview.com

It is not as neat as the built-in viewer, but works for my needs.

Thanks for your help