Check the following hidden preference:

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. Locate the preference keyword.URL, if it shows Yahoo right-click on it and choose Reset, this will clear it and it will use Google instead.

If that preference doe not show Yahoo, you will probably have an add-on that is causing this. To identify what add-on you can use the procedure in this link - https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Now the search is not working. It is directly opening the website. Like if I enter "test" in address bar and press space bar and then press enter, I am directed to http://test/. I have set "http://www.google.com/search?q=%s" and restarted the browser. It is not working, I have also tried "http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=" but neither worked for me. Please tell me if I making mistake in the string part. Only the improvement is that the yahoo is not opening.

In about:config check the preference keyword.enabled, it should be set to true, if it is not double-click on it to change its value.

Yes dude keyword.enabled was already true. I am sorry to specify that in my last response. I guess there is anything wrong with my search string which I ma setting for keyword.url

This issue can be caused by an extension.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

In Firefox 4 you can use one of these to start in Safe mode:

• Help > Restart with Add-ons Disabled
• Hold down the Shift key while double clicking the Firefox desktop shortcut (Windows)