Before today I didn't know they existed, but after receiving an email from a friend telling me about them, I had to try them out, and I can tell you this much, they are great!
Firefox Keywords Shortcuts: Type in a few letters in the location bar and you'll jump right to your favorite search engine or directly to a search results page for your search of choice.
An example is my bookmark for the English version of Wikipedia to which I have added the keyword "wpen". With that set, I can type "wpen" in the Location Bar and it loads the page
To set a keyword you need to create a bookmark for the URL. Next you add the keyword to the bookmark.
Open the Bookmark Manager (Ctrl+B), choose your bookmark, and then open the Bookmark Properties window. In the window simply add a short string to the Keyword field. Now close that dialog and you can type the keyword in the Location Bar, hit enter, and the browser will load that URL.
But that's not all! Keywords Shortcuts can be used to create shortcuts for your favorite search engines too.
Lets say you want to add a keyword to do a search on Google. Bookmark the address:
http://www.google.com/search?q=%s, next add the keyword "goog", or you can even add a simple question mark "?" as the keyword.
If you used the question mark as a keyword, all you have to do to make a search on, say "indian spices", is to type the following in the location bar of the browser: ? indian spices and you will jump right to the search results from Google.
You can do the same with search results from Wikipedia. In the above example where I used the keyword "wpen" you can change the address of the bookmark to point to
If you want to search on Wikipedia for the "moon", you can do so by typing the following in the address field:
wpen moon and you will jump directly to the search results.