When you click on the address bar in Firefox 3, instead of placing the cursor at the location of the click, it selects the entire URL in the address bar. This behavior is controlled by the browser.urlbar.clickSelectsAll preference.

To change this, type about:config into the address bar and search for the preference called browser.urlbar.clickSelectsAll. Once you find it, double click on it and the value will change from true to false so that clicking in the address bar no longer selects the entire address.

This preference is not new to Firefox 3, it exists in Firefox 2. What has changed is the default value. So if you want to change your behavior in Firefox 2, you can use this same trick to enable/disable this feature.