Change the default search engine for Firefox's address bar

In FireFox, in your address bar type

about:config

and press Enter. Now look for an entry that's titled "keyword.URL", this is the config key that contains which search URL to use when you search through your address bar. If you double click the entry, you can change the value.

By default it should be set to google, which uses this query URL: http://www.google.com/search?&q= It's possible it's a bit different but it should be similar.

Some examples for search engines' query URLs:

Google: http://www.google.com/search?&q=
Yahoo: http://search.yahoo.com/search?p=
Ask: http://www.ask.com/web?q=
Bing: http://www.bing.com/search?q=1

If you want an other search engine, and don't know the query URL, do the following:

1. Go to your preferred search engine (E.g. Google)

2. Perform a search with an easily recognisable keyword like "HELLOWORLD"

3. In your address bar, there should be an URL containing your keyword. For Google it would be something like http://www.google.nl/search?q=HELLOWORLD&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:nl:official&client=firefox-a

4. Now just delete everything after the "=" that's just before your chosen keyword. Your query URL for Google would be: http://www.google.nl/search?q=

   

Starting with version 34.0.5, this can be changed by going to Options Search

_{(source: mozilla.net)}

It can also be changed programmatically, although this is more difficult. The search engine setting is now kept in the Profile Folder, in a file called search-metadata.json. Example

{
  "[global]": {
    "current": "Google",
    "hash": "fGiy1O53LgQtereW/4qUSlwFfagTNKGghURIbAFw8wY="
  }
}

The hash can be created by using this example Bash script

profile=default
provider=Google
disclaimer="By modifying this file, I agree that I am doing so only within \
Firefox itself, using official, user-driven search engine selection processes, \
and in a way which does not circumvent user consent. I acknowledge that any \
attempt to change this file from outside of Firefox is a malicious act, and \
will be responded to accordingly."
printf "$profile$provider$disclaimer" | openssl sha256 -binary | base64

This change was discussed at Bugzilla and can be found in the source code.

I had this problem when the default search UI changed in the latest version of firefox. When I changed the default search in the search bar, the search from the address bar also changed.

For instance, this...

Results in this...

However if you want to choose which engine to use each time, then you should go with the solution from @harrymc

I hope this helps.