<p>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)</p>
<p>now go to firefox's search settings. you can get to it by putting the text below in the search bar:</p>
<code>about:preferences#search</code>
<p>the table with all of your search engines should now have a new button below it that says "add"</p>