For example to search on Google "g search keywords", on Wikipedia "w epr paradox", Dictionary "d prometheus". Opera and Chrome have this too IIRC.
To search history "^ page keywords", bookmarks "* page keywords", tags "# tag words" (there are others, I blogged about them http://alicious.com/fast-bookmark-and-history-search-in-fire... or see http://kb.mozillazine.org/Location_Bar_search).