There are some sites I visit a lot, like the Ubuntu Forums. A friend recently showed me this trick, which I really like, so now I'm showing you. Just bookmark a page and then go to Bookmarks -> Organize Bookmarks. Right click on the bookmark and hit "Properties". You'll get this:
See the keyword field? I put "uf" in there. Now I can just type "uf" in the address bar and Firefox automatically takes me to the forums!
I was also just reminded that you can put something like this...
And then type "gg ubuntu" in your address bar to get the results for a Google search for "ubuntu". The %s inserts whatever string you put in into the search.