Dictionary.com search plugin
Create a file called "Dictionary.src" in your Firefox searchplugins directory containing the following:
description="Dictionary Search "
<input name="q" user="">
charset = "UTF-8"
Now select "Dictionary" in the search bar and type your search term.
If you want to get more technical with this sort of search plugin check the documentation.
Dictionary.com bookmark keyword search
I prefer typing to mouse clicks and so I generally keep the search plugin for google. I have a bookmark setup for Dictionary.com with a URL of http://dictionary.reference.com/search?q=%s and a keyword "dict" (you need to go to "Bookmarks"->"Manage Bookmarks" and then "Properties" to edit the keyword value) then to search for "chiaroscuro" go to the nav bar and type
and hit enter.
Perl programmers might find it useful to have a keyword search for cpan:
More on bookmark keywords - *update:* looks like AOL pulled the plug on devedge try this webarchive copy instead.