This thing has annoyed me for months now! When typing a keyword in the default GoogleBar in Firefox, my results were displayed using the google.com page.
Living in Belgium, I’ve looked many times in Firefox preferences and in about:config for a parameter to change that option… No way!
Finally, I found this website allowing you to add about any search engine you want, google.be included: mycroft.mozdev.org. Simply add your parameters (country, language, … ) and click on the desired engine to add it to the list. Then select it and start searching!
In my particular case, I had the choice among those search engines.