Q: How do I tell Firefox to select a certificate automatically?

Why am I being warned that "This site has requested that you identify yourself with a certificate" when I visit a web page the requires an MIT personal certificate in Firefox 2.0.0.13 or later?
In Firefox 2.0.0.13 and later, why am I being asked to "Choose a certificate to present as identification" when I visit a web page the requires an MIT personal certificate?
Why does Firefox keep prompting me to select a certificate rather than just presenting one automatically?

Context

Firefox 2.0.0.13 or later
Personal web certificates
X.509 web certificates
MIT servers requiring user authentication

Answer

Firefox 2.0.0.13 changed the default behavior of personal certificates to prompt the user each time a web site requests a certificate. The old behavior of not prompting a user made it easier for a malicious web site to track users activities by requesting the client certificate, even though they were from a different domain.

To proceed when Firefox asks when an MIT web site asks for a certificate

When you see the message "This site has requested that you identify yourself with a certificate..." click on the Ok button. Firefox will then present your web certificate to the web site requesting it and authenticate you. If you see this request from a non-MIT site, make sure you trust the site before clicking okay.

To change the default behavior to present a certificate automatically

You can revert to the old behavior by following the below instructions for your platform.

Windows:

Go to the Tools menu and select Options
Select the Advanced section in the Options window
Select the Encryption tab
In the Certificates section on this tab select the radio button next to Select one automatically

Macintosh:

Go to the Firefox menu and select Preferences
Select the Advanced section in the Preferences window
Select the Encryption tab
In the Certificates section on this tab select the radio button next to Select one automatically

Linux:

Go to the Tools menu and select Options
Select the Advanced section in the Options window
Select the Encryption tab
In the Certificates section on this tab select the radio button next to Select one automatically