OMG! TIL there's a way to add back the long-since-removed capability to add your own custom search engines to Firefox.
In about:config, add a new boolean key browser.urlbar.update2.engineAliasRefresh = true.
Then the "Add" button is available in search preferences (put %s where the query term goes).
Source: https://superuser.com/questions/7327/how-to-add-a-custom-search-engine-to-firefox/1756774#1756774