enter the text below in firefox's search/url bar:
about:config
in the text field that says "search preference name", enter the text below:
browser.urlbar.update2.engineAliasRefresh
make sure "boolean" is selected and press the + button on the right. it should automatically set the preference to true. if it doesn't, press the toggle button on the right (looks like 2 arrows pointing opposite directions)
now go to firefox's search settings. you can get to it by putting the text below in the search bar:
about:preferences#search
the table with all of your search engines should now have a new button below it that says "add"