How can additional sites be added to the Firefox search engine?

And by that I mean, is there a way to literally type (or paste in) a new site to the firefox search engine?
I’m aware of the fact that whenever you navigate to a site that CAN be added to the engine, it will glow and let you know that that site CAN be added.. and so it gives you the option to do so.
But what if I want to add another site for which the engine does not glow..?
I currently have the following list added to the search ability:
Last.fm
IMdb
Facebook
Google
Dictionary.com
Yahoo
Amazon
Answers.com
Ask.com
Creative Commons
Ebay
Wikipedia
WOT (the safe search add-on)
Youtube
Firefox Add-ons
Photobucket

And I’d like to add some more:
Etymonline
Archive.org
Fluther
Ehow
AMG

and some others that don’t come to mind as of now.

If I open the “manage search engines” window, I sadly notice that it does not give me a type-in option to add new sites.