If you arent concerned about someone looking over your shoulder and stealing your passwords, why hassle with those obfuscated password fields, where you never know whether you typed your 30 character code correctly or not...
Unhide Passwords is a Firefox extension that shows the contents of password fields in clear text (instead of the asterisks), to make that process a bit easier.
Due to popular "demand" the double-click feature has been removed again from V1.2.1 on.