Just slightly ahead of the release of Firefox 8, JSONView 0.7 has been approved on the Mozilla Add-ons site. Firefox 8 actually handles JSON documents in the browser for the first time, reducing the need for extensions like JSONView - now, a vanilla Firefox will display JSON as plain text instead of prompting a download of the document. Still, JSONView provides much more than what's built in, and it will continue to be compatible with Firefox going forward (and it's still compatible with older versions all the way back to version 3.0). If you're on a modern version of Firefox you've probably already updated, and if not, you'll get the new version when you upgrade.
After this update, I'll be starting work on version 1.0, which should include some power user functionality that a few people have asked for, as well as some infrastructure changes that modernize how JSONView works with Firefox. Please fork the code on GitHub and contribute if you have something you want to add!