This add-on for Firefox and Edge adds a page action to the address bar for pages which contain valid JSON, enabling you to pretty-print the page contents with a single click.
Unlike similar add-ons, this add-on does not add a fancy editor or colors or collapsibility or anything. The page after using this formatter remains valid JSON that can be copied and pasted somewhere else; it simply contains whitespace after formatting.
Source for the MS Edge version is available in the ms-edge branch.