text/jsonl is a common spelling for JSON Lines (e.g. ServiceStack serves it) that bug 2055774 did not cover, so such responses fell through to the plain text path instead of the JSON Viewer. Accept it in the sniffer alongside the application/ spellings, and in fixSave() so Save As restores the served type rather than falling back to application/jsonl. Differential Revision: https://phabricator.services.mozilla.com/D317542