Launching Thunderbird on Windows or Linux for the first time should generate a prompt asking you if you want Thunderbird to be your default email client. If you happen to answer "No" and you want to go back later and make Thunderbird the default, follow these steps:
If at some point you switch to another email client and you want to go back to using Thunderbird, the next time you launch Thunderbird you should get a dialog box asking you if you want to make Thunderbird the default. At the time of this writing this is not supported on the Mac due to the way Apple manages the default email choice through mail.app. To set Thunderbird as your default, you actually have to set up a mail.app account first and then go into mail.app Preferences to make Thunderbird your default email client.
If you encounter problems with mailto: links not launching when you're using Windows, you might have to go to Start | Settings | Control Panel and open the Internet Options tab. Go to Programs and select Thunderbird from the dropdown list under E-Mail.
You've taken the first step and downloaded Thunderbird. An exciting world of mail management awaits you. Let's move on to the next chapter and show you how to set up your mail, RSS, and news accounts in Thunderbird. You'll quickly see how Thunderbird will allow you to soar to new heights!