Firefox Search Engine Plugin
Simply click on this link: Massachusetts Library Directory Search Plugin for Firefox & Netscape
To uninstall a custom search engine, simply select "Manage Search Engines…" from the Search Bar, select "Mass. Libraries" and click "Remove". If you have Firefox version 1.x , follow the instructions given at MozillaZine. You must delete the files from your Firefox "Profiles directory".
To uninstall standard search engines from Firefox, first install the SearchEnginesHack Plugin from Firefox Add-Ons or Windows Search Plugin Manager, which is a separate Windows executable. You will then be able to right-click on the Search Engine you want to remove and delete it.
For more general information about Firefox/Netscape/Mozilla plugins, visit MozDev.org.