"Add more search engines..." should be replaced by a text and two links
specifically to mozilla website
https://addons.mozilla.org/en-US/firefox/search-tools/
and also to mycroft project
https://mycroftproject.com/
(that i think is far superior in this regard)
I looked into this and it does look like a good idea! :-)
Here's some additional information that, I hope, can/may/could be helpful:
https://addons.mozilla.org/en-US/firefox/addon/mycroft-project/
https://github.com/Smile4ever/mycroftproject-query
~Ibuprophen
Otherwise you can also use the :
From my point of view, the syntax is very simple to use and to remember.
For the record :
## In this example, it will replace the default search engine (Google by DDG).
defaultPref("browser.search.defaultenginename",'data:text/plain,browser.search.defaultenginename=DuckDuckGo');