English is not my first language. Often I open urls http://dictionary.reference.com/ and http://thesaurus.reference.com/ to get a more information.
The Mycroft project provides a collection of search plugins which allows you to access a search engine right from your browser. In Mozilla Firefox, you use the search box on the toolbar including Dictionary.com plug-in :)
Mozilla has put a list of few popular search sites in the Search Bar in the upper-right corner of Firefox. If you'd like to add more, there are hundreds to choose from. Click on a Search Engine to add it to your Firefox Search Bar.
Now I can search directly from Firefox :D