There is a better solution... Rated 1 out of 5 stars

Don't bother with this or any other search bar plugin.
Instead, install the "Add to Search Bar" add-on by Dr. Evil and you will be able
to add ANY search engine you'll ever come across to your Firefox search bar.
Just right-click in its search field and choose "Add to Search Bar".
Easy as that.

Get it here:
https://addons.mozilla.org/en-US/firefox/addon/add-to-search-bar/