Hi Peter,
You can use this page: http://mycroft.mozdev.org/submitos.html at Mozilla. It provides a web-based form for setting up search plugin frameworks. It's mostly a "fill in the blanks" form, with the trickiest part being the search URL and suggestions URL fields. You'll need to do some experimentation with those because each site is somewhat unique in how it's searched. Be aware as well, the site you want to search may have restrictions. You'll have check it out ...
You might find this site: https://developer.mozilla.org/en/Creating_OpenSearch_plugins_for_Firefox useful for understanding the plugin structure.
Once your plugin is debugged, you can add it to the Mycroft database. A good thing to do because if you decide to modify it in the future, maybe a URL changes for instance, you can reload it and just make the appropriate changes.
I'll be out for the week with no internet access, leaving this afternoon. If you run into trouble, please PM me or post here and I'll get back to you as soon as I get home.
Cheers!