Doesn't work for me. Ask.com has overtaken Google as my default search. I've gone into About:config and tried many of the suggested approaches. No luck so far.

Still a problem when using CTRL-K, when using a right click on a string search, and when using the Tools menu drop down.

Tried changing the about:config. It takes, but does not change what is happening. Checking the about:config after a reboot and the google search url is their - just not being used.

The Preferences not saved issue does not help either.

Currently stuck with yahoo, unless I go to the navigation menu bar.

instead of

http://www.google.com/webhp?ie=UTF-8&oe=UTF-8&sourceid=navclient    
just reset , it goes back to google

Thanks very much. It worked!

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