You're sure its not the right hand side? (Chances are I'm wrong so just ignore me.)
I doubt you mean getting rid of certain one, because all you have to do is choose the one you want and leave it. If you don't wanna view it, go to View-Toolbars-and uncheck navigation toolbar.
Go to C:\Program Files\Mozilla Firefox\searchplugins
Each search engine will have two files; a .src and either a .png or a .gif file. Just delete the two files for the search engine you are trying to remove (for example google.src and google.gif) and then restart Firefox.