Mozilla have not exactly gone out of their way to promote this feature but it sure is useful. Keyword searching makes performing common searches on websites much faster by enabling you to create a 'keyword' that associates the search url to what you type in the address bar. It is not exactly the best description but it means that rather than visiting a site to perform a search (say for a recipe or book on amazon) you can assign a keyword like 'recipe' and then in the addressbar just type 'recipe roast duck' to have Firefox perform a search in your favourite recipe site.
It is a very nice touch and has definitely increased my browsing productivity of late. It seems like such a shame it seems to be slightly buried in the interface and slightly vaguely worded.