SearchPluginHacks is a Firefox extension which can help you remove Search options.
It seems kind of weird that this isn't included in Firefox, but it's also strange that the developer of this plugin didn't mention that if you're simply wanting to remove some of the search engines which come installed by default, then you can just delete the .src and the graphic file from your "searchplugins" directory (on Windows, C:\Program Files\Mozilla Firefox\searchplugins, or whereever you've installed Firefox).
Technorati tags: Firefox, Web Development