You can still disable the search engines if you have root access to your device. Specifically, you need to pull and modify the file search.json. On my device, I have:
adb pull /data/data/org.mozilla.firefox/files/mozilla/rbn69ru0.default/search.json
Now, by default, this file doesn't format everything nicely, so you can use something like
in order to clean things up. In any case, you'll see bits such as
You need to take the "_hidden" flag and change it to true. Do this for all of the search engines to hide all of them. Or, frankly, just delete everything under engines. Then, upload the file back and restart firefox with a command like:
adb push search.json /data/data/org.mozilla.firefox/files/mozilla/rbn69ru0.default/
Until an application restart, the changes won't be seen.
As a slight editorial note, it's dumb that we have to do this by hand. Either we should be able to disable things under Settings->Customize->Search settings or we should have some way of turning things off in about:config. While I understand that removing menu items that cause too many support issues, completely removing the option even from about:config is counterproductive.