Fabio Said has a post (Installing many dictionaries as Firefox and Internet Explorer add-ons) in fidus interpretes about installing dictionaries, encyclopedias and other reference works as search engines in Firefox or other browsers. An excellent tip that I'm sure will prove useful to many of us. Check it out!
Beside the technique suggested by Fabio, you can set up Firebfox (or IE7) so that it opens with several dictionaries, each in its own tab. For example, I have tabs for the Garzanti, Rizzoli and Paravia English-Italian dictionaries, as well as separate tabs for the De Mauro Italian dictionary, for the Simone legal dictionaries, and for the English and Italian Wikipedias.