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
@dalias I haven't played with it in a while, and I believe it was also possible to set it from a website in JS with a xml config
@webhat Yes, but that's awful for anyone who doesn't make websites. No way to trust that 3p ones just do a url with pattern subst without leaking tracking info and shit.
@webhat This was one of the most viscously user hostile changes Mozilla ever made. Taking away a simple tool even normies could understand to customize search to meet their needs, and replacing it with "trust the big websites, lol".
@webhat And it's even more insulting to users that the code is all still there, just hidden behind an about:config var, and that they went to the trouble of making it invisible by default so you have to explicitly add the key, just as an extra fuck-you.
@dalias absolutely, that's what I meant with that I believed the feature was still there to be accessed from a webpage with JS. I'm sure that there are many other controls which are only accessible from webpages and no longer from the UI