Unhide Password is a simple Firefox extension that reveals all passwords by displaying them in plain text instead of the normal asterisks.
Note that the password is only visible when the password field has focus. This is useful while filling a long form or while dealing with already saved passwords, since you only need the password to be visible when typing it.
You can also opt for “semi-privacy” by setting the color of the password to a light shade so that only you can see the password.
If you don’t want to install an add-on, you can use this bookmarklet for the same functionality.