Searching using the default Google search from Firefox's address bar will use Google US, but if you want to change this to Google UK, you can use the following method.
You can edit many of the Firefox's preferences by opening Firefox and typing "about:config" (without the quotes) into the address bar. You will then be greeted by a screen that reads "Hear be dragons!". Click the button that reads "I'll be careful, I promise!" (and make sure you are), and the preferences list will appear. As you can see, there are loads. Type "keyword" into the "Filter:" search field at the top of the page and two preferences will be displayed, "keyword.URL" and "keyword.enabled".
Firstly, copy the line of text above and then double click on the "keyword.URL" preference. A pop-up box titled "Enter string value" will appear. Paste the text into the input field and click "OK". Next, if the text in the "Value" column of the second preference "keyword.enabled" reads "false", double click on the preference to change it to "true". This will allow the search keyword we just set to be used. With this setting set to "false", any search in the address bar will take you to the closest URL to your search term. You may like this functionality, but the sites you will be taken to will only be in relation to the URL, not in relation to any search engine page-rank or site quality.
Close the "about:config" tab or Firefox itself to close the preferences menu. Other resources on Firefox include my blog post How to view your Firefox bookmarks in a tab and MozillaZine's About:config entries article.