The preference
browser.search.defaultenginename controls which search engine is displayed by default in the search bar when you start Firefox and the value is a name corresponding to one of the engines available in the searchbar list, not an URL. Besides, this only works if
browser.search.selectedEngine is the default value, which means once you select a new engine from the list, Firefox will always start using that engine, even if the default engine is a different one. Please notice this doesn't affect the default engine of the address bar.
If you want to change what search engine Firefox uses when you type something in the address bar, then you have to modify the preference
keyword.URL. Nevertheless, Google redirects the query on the server. This redirecting "feature" seems to be inconsistent, since sometimes I get google.com, sometimes I get google.com.br.
If you want to modify the Google search engine on the searchbar, install
Add to Search Bar, then visit
http://www.google.co.uk, right-click on the google search field and select "Add to Search Bar". You might want to delete the old one first.
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