Type about:config into the location bar and press enter

Tried that, doesn't work.

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Thank you.

The keyword.URL preference is no longer used in Firefox 23 or above and setting the engine for address bar searches is now done from the Search Bar -see Search bar - add, change and manage search engines on Firefox. As a workaround you can use the Keyword Search add-on.

If you have the Yahoo Toolbar installed then see also: Remove a toolbar that has taken over your Firefox search or home page.

@AliceWyman sorry i forgot that, i just check his OS it was earlier version of windows, and i was on bus.

I think he got what i meant.

I wonder how "Resetting Firefox" would benefit as none of those about:config (Yahoo) settings were ever changed. I see some Yahoo URLs are hard-coded, they cannot be reset or altered. With every update of Aurora or Nightly, the Yahoo settings are restored and the default search engine is set to Yahoo.

Its weird no one suggested the Search Reset Tool to reset only the search prefs.

"This add-on is very simple: on installation, it backs up and then resets your search preferences and home page to their default values, and then uninstalls itself. This affects the search bar, URL bar searches, and the home page."

Interesting add-on. I will try this on the affected machine and in the browser version that refuses to be 'reset'. Thanks for your suggestion. By far the best suggestion so far.

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved

Thank you, Cor-El

You're welcome