Allows rapid access to (my) favorite search engines. Just type in address bar g something to search something with google. It's useless in FF 2.0. Read below why.
g - google.com
w - en.wikipedia.org
wpl - pl.wikipedia.org
a - allegro.pl
e - e-dict.pl (PL/EN dictionary)
Thank you for comments. Especially for:
"Right click in any website's search field, and you can create this same functionality."
In older versions than 2.0 I didn't found this functionality and editing bookmarks and assigning letters for search engines was too difficult, i.e. for ma grand father :)