+++ This bug was initially created as a clone of Bug #1387650 for macOS +++ Firefox is the best application to view JSON files on many Windows installs since bug 1243951 but it doesn't show in the Open With options by default since it doesn't advertise that it supports JSON files. Firefox should advertise that it's a viewer of JSON files.

I'm probably not going to be working on this at least anytime soon, but I'd be happy to take a patch for it. File associations are managed by a couple of functions in /browser/installer/windows/nsis/shared.nsh, so that's where the main change will need to be made. The uninstaller (/browser/installer/windows/nsis/uninstaller.nsi) will need to be updated as well.