Just type in your search terms and hit Enter button in the address bar of your Firefox browser URL. Firefox will take you to the Google search results page or Yahoo or bing search results pages.
You can also do the following :
- Web search in the address bar
- Turning off the web search in the address bar
- Changing the search engine for the address bar
- URL autocomplete
- Domain Guessing
- Turning off Domain Guessing
- Changing the prefix or suffix.
Read all these in the Mozilla blog
Website Address: http://support.mozilla.org/en-US/kb/search-web-address-bar