From the linked Librewolf FAQ anyway:
Please note that some search engines, most notably Google, do not comply with the OpenSearch standard so you will not be able to add them the usual way. In that case, you can also check The Mycroft Project for an alternative source of search engine plugins.
Anyway, I think here we’re going against the actual philosophy of Firedragon, being a privacy-oriented browser. So I, for one, give up investigating further…