For users of the Mozilla Thunderbird email client you might find this How-To useful for replacing MapQuest.com as the default map website in the application's address area of the card summary pane.
Chris Ilias has developed these instructions to replace it. Here they are from his blog post on the topic:
It would probably be best to use the AboutConfig extension; so if you don't already have it installed, install it.
Using the AboutConfig extension, look for the preference:
Double-click on it, and edit the value to:
For more info see this link.