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To change your default browser, launch Safari and go to the Preferences. In the General pane, there's a place to select your default browser from a list of installed browsers on your machine.
Likewise, to change your default mail client, launch Mail.app and go to the Preferences. You can select from a list of mail clients or choose one not on the list by clicking "Select...".
For versions of OS X prior to Panther (10.3.x), the default browser and mail preferences are set using the "Internet" pane in System Preferences.