How to change the default searchengine in mozila?

SOLUTION NO 1.
In Firefox type about:config in the address bar and press ENTER.
Search for Keyword
Locate and double-click the entry for keyword.URL
Modify it by add the below set of values :
Yahoo: http://search.yahoo.com/search?p=
Ask: http://www.ask.com/web?q=
Bing: http://www.bing.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=
Google: http://www.google.com/search?&q=
DONE

 SOLUTION NO 2.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences
3. In the filter box type keyword to bring up a small number of preferences
4. Right-click on the preference keyword.URL and choose Reset to set it back to Google