It works... Not as expected, but it works... I expected it to be integrated with Firefox, opening a JSON file directly from it, or entering edit mode on an open JSON (since Firefox 53, JSON files are natively viewable in it, so there's no need for JSON viewer extensions.) Long story short, it doesn't. Not only that, but it also cannot load or save JSON, it literally is a JSON editor, ONLY an editor, without any other functionality, not even load/save! The JSON data must be copied and pasted into the editor window and, after any editing is done, the code copied and pasted back out into a JSON file. The editor is not even good (no all tree expansion/collapse, so it has to be collapsed/expanded manually, one by one...)