Actually i faced this problem after installing DAP. My default address field search changed to some speed bit search and it was just a mess and i was not able to remove it so after doing some hunting got a solution!
This is how it works.
In the URL field type about:config and press enter.
Find the key called keyword.URL and double click it. Give your search engine query or url and its done.
How to get the url?
Here is how i get.
Say for example yahoo search you want for default browser.
–>Go to Yahoo.com
–>In the search field type any single word and enter. i gave HI
–>If you see the URL you get something like this.. http://search.yahoo.com/search?p=HI&…p=mss&ei=UTF-8
–> if you see you can find something like this http://search.yahoo.com/search?p=HI&fr=y
–> copy till HI.. so you get http://search.yahoo.com/search?p=
–> copy it and paste it in keyword.URL and yahoo! you default search changed to yahoo..
Similarly if you want live search just paste http://search.live.com/results.aspx?q= and you are done.
For changing back to default google.. paste http://www.google.com/search?btnG=Google+Search&q=
or this if you want I AM FEELING LUCKY feature of google to work!