For wikipedia, click on the icon in the search box, select 'Add engines' at the bottom of the list, and then click on wikipedia from that page to install that search plugin.
The other two don't seem to be available as plugins, but you do have a different option: You can add them as a quick search. A quicksearch is essentially a keyword you can use in the address bar which is linked to a search on a specific site. Anything following the keyword is then treated as the search term.
I'll walk you through how to set it up for Google Maps, and the same method should work for everything else:
1) Go to the main page at maps.google.com
2) Right click on the google search box (not the one in firefox)
3) Select 'Add a keyword for this search'
4) Pick a name for the search (we'll use 'Google Maps') and a keyword (we'll use 'map')
5) Place the bookmark you just created in the quick searches folder, or if you put it elsewhere use the bookmark manager to move it there
6) Open the properties of the new bookmark and delete everything following the query, in this case everything following 'maps?q=%s'
7) Type 'map searchterm' into your address bar, and it'll search as if you went to the page and typed 'searchterm' there.
[Edit] Actually, I can't get this to work for encyclopedia mythica. But google maps definitely works.
[Edit again] Cracked it. FF apparently won't set up a quicksearch on it's own for pantheon.org because it uses a cgi script for the submit, or something, but you can still do it manually and have it work.
1) Make a new bookmark, anything will do, and put it in the quick search folder from before
2) Go into it's properties and paste 'Encyclopedia Mythica' into the name box, '
http://pantheon.org/cgi-bin/search.pl?nocpp=1&Realm=mythica&fa=0&Terms=%s' into the location box, and 'myth' into the keyword box
3) Click ok
You can now seach encyclopedia mythica from anywhere using the 'myth' keyword in your address bar.
[And again...] Figured I'd post some of the quicksearches I have set up:
GameFAQs
Name: GameFAQs
Location:
http://www.gamefaqs.com/search/index.html?game=%sKeyword: faq
IMDB
Name: IMDB
Location:
http://www.imdb.com/Find?for=%s&select=AllKeyword: imdb
Wikipedia
Name: Wikipedia
Location:
http://en.wikipedia.org/wiki/Special:Search?search=%sKeyword: wiki
Dictionary.com
Name: Dictionary
Location:
http://dictionary.reference.com/search?q=%sKeyword: dict
[Bleh] And of course after all that, it turns out there *is* a search plugin for pantheon.org. In the most unlikely of places no less, namely on pantheon.org... get it
Here. Still, the keyword search is generally more handy anyway, so not a complete waste.