This is something I wanted to figure out for a while, and once I sat down to actually do it, it took about 2 minutes. Not only can you remove the shortcuts to the search engines, you can add search engines too. For me, the fact that Amazon search was there while MSN search was not was a pain. If you are a user of the search facility in Firefox and you want to be able to customize the search engines that appear there follow these simple instructions.
To Remove Search Engines
Go to C:\Program Files\Mozilla Firefox\searchplugins\ and delete the search engines you don’t need. There are two files which need to be removed for each unwanted search engine, the shortcut file and a .scr file.
To Add Search Engines
Visit here and click on the search engines you would like to have. Shortcuts for everything from BBC to Technorati are easily installed by just clicking on the relevant link.