If you want to add a custom search engine, for example your own searx instance, do the following:
Open the url
about:config
(If a warning shows up, klick 'Accept the Risk and Continue')
Type
browser.urlbar.update2.engineAliasRefresh
in the search box
Click on the '+' symbol
Open the url
about:preferences#search
(Go to search preferences)
Under the Search Engines ListBox there is now an 'Add' button, press it
Fill in the Form, add an optional keyword (eg. s search-term)
Optional: Change to your custom search engine under 'Default Search Engine'
Enjoy (Why do they hide this?)