Useless.

Was useful until Firefox 53, with the arrival of its own (and better) json viewer.

This extension is not as good as the json viewer that's built into Firefox since v53. See https://developer.mozilla.org/en-US/docs/Tools/JSON_viewer

Firefox already has excellent built-in support for viewing JSON files. (Currently, I am using Firefox 65.) This add-on is a serious regression in comparison.

...actually Firefox has always rendered JSON beautifully without the need for any plugin.