Replying to self here, mysteriously, the viewer is now working again.

I spoke too soon. This looks to come and go. This morning with a fresh reload of Firefox it's back to not displaying and throwing the 'you have chosen to open' dialog again.

Is no one else experiencing this?

The builtin JSON viewer only works with local .json files and files send by a server as application/json. Any other file type will give a download dialog that you would have to override to make the JSON viewer work.

• https://developer.mozilla.org/en-US/Tools/JSON_viewer
• https://addons.mozilla.org/firefox/addon/content-type-fixer/

I don't understand, this was working in the last version. It was working intermittently, then it stopped working.

Adding the Content Type Fixer add-on had no effect, json was set to ON.

Does the file have a .json file extension ?

Try to rename/remove handlers.json and delete mimeTypes.rdf, when present, in the Firefox profile folder with Firefox closed to reset all file actions.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/Profiles

Yes, the files have .json extensions.

Deleting both files with Firefox closed didn't change the behavior unfortunately.