If you aren't 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...
This extension shows the contents of password fields in cleartext (instead of the asterisks), to make that process a bit easier.
If you want "semi-privacy" (where *you* can see the passwords, but any bystanders cannot), you can try setting your password color to something low-contrast (like light gray for example).
(If you are having problems with this extension not working with certain sites, then that has most likely to do with the pages not having finished loading yet. Only pages that have been fully loaded can be modified by addons, so in case a site in general, or parts of the page, are very slow, you will still see the encrypted passwords until the page has finished loading.)