Let me give you an example to clear things up. You may right-click any form field in Firefox and select the menu entry Add a Keyword for this search. Selecting this opens a new Add Bookmark menu which asks for a name and a keyword. I went to ebay.com and did a right-click on their site search. I entered the keyword “e” and the name eBay.
Typing in “e computer” would have the effect that a search on eBay would be performed with the search string computer. I only have to type this phrase into the address bar to perform the search. The same method works for other websites including, Google, Yahoo, IMDB, Deviantart, Myspace and Slashdot.
Last but not least you can add wildcards to urls as well. If you edit a bookmark you see that it asks for a location and a keyword. The locations is normally the url of the website and the keyword the shortcut key. A wildcard would be added to the location to further customize a search or visit a special part of a website.
The wildcard symbol is “%s” which means that a valid location would be for instance http://%s.google.com/. If g would be assigned as the keyword you would be able to access Google websites such as adsense (adsense.google.com) or video (video.google.com).