Step 1: Basic keyword bookmarks
When you add a bookmark, it's as simple as typing the keyword into the "Keyword" section of the window. Here, I've used "engad".
Now, when I type the word "engad" into the address bar, Firefox will go straight to engadget.
Step 2: Searching with keyword bookmarks
To do this, right click on the field that you want to add your keyword to, and click "Add a keyword for this search". Then enter the name of the page (Not massively important but it helps keep your bookmarks organised) and enter the keyword for the search, in my case, "wiki".
Now when I type "wiki searchingforsomething" Firefox will search Wikipedia for "searchingforsomething". This can be useful because you can use your address bar instead of the Search bar, and hide the search bar, allowing more screen real estate.