Checked into yesterday's (2004-07-17) aviary (Firefox 1.0) branch: an option to show/display saved passwords.
To check it out, grab yesterday's (2004-07-17) nightly branch build, go to Tools -> Options -> Privacy, and select the Saved Passwords option.
Click on View Saved Passwords to see the Password Manager and you'll see a Show Passwords button at the bottom right. That's new.
You get a confirmation dialog (supposedly because showing your passwords is an important event). I think the intended use of the confirmation dialog is to save you from inadvertently showing your passwords to someone looking over your shoulder.
And your passwords are then shown in the plain text glory (mine are "pinked out" in the screenshot below, of course).
Looking at my list of passwords, I am beginning to think I'm not a very secure person (identical passwords for different accounts, using the same passwords for long periods, and passwords shorter than 8 characters).