Download an existing 'opensearch' XML file for one of the DuckDuckGo provider add-ons, such as the 'Lite' version (or the 'html' version which is also non-JS by default): https://addons.mozilla.org/en-US/firefox/addon/duckduckgo-lite/ or https:// addons.mozilla.org/en-us/firefox/addon/duckduckgo-html/. Do it by right-clicking on the 'Download Now' button on the add-on page and 'save link as' to save the .xml to your computer.
Modify the XML on your computer to replace all instances of
3g2upl4pq6kufc4m.onion. For extra security and privacy, double check that https is included in the url strings - HTTPS works on .onion urls also.
<ShortName> line, modify the name if you want to differentiate it from the existing one or just make it clear to yourself it's the Tor DDG. e.g.:
<ShortName>DuckDuckGo Lite Tor</ShortName>.
<Param name="kp" value="-1"/>.
Now upload the XML file to a temporary file sharing service that can provide a direct http link of your modified XML file. E.g.: https://gist.github.com and it's Raw-Link.
Find a 'HTML sandbox' site that provides a template for a browser-readable HTML page (a great one that requires no JS and is on HTTPS is https://www.jmarshall.com/easy/html/testbed.html), and put in the
<head> section this tag:
<link rel="search" type="application/opensearchdescription+xml" title="DuckDuckGo Lite Tor" href="https://direct-link-to-your-modified-xml-file.xml">
Load the html sandbox code, and a blank page will be the result (but code obviously in the source). On that page, the tag tells Firefox that a custom search engine is to be found on this page! Go to the search bar and use the drop-down arrow on the left, and you'll see 'Add "DuckDuckGo Lite Tor". Do it and you can now search from the Firefox bar with Tor hidden service anonymity, forever!
FYI, the info on adding the tag was found at: https:// developer.mozilla.org/en-US/Add-ons/Creating_OpenSearch_plugins_for_Firefox
Interestingly, this method works for adding ANY custom search engine XML to Firefox in general, which I have also been trying to know how to do (without editing tedious XML/json files in the profile folder and having to restart Firefox, yuck) for years and years, and now I know, boo-ya!
Of course, edit the title or version of DDG as you see fit, my guide is merely a template.