This extension makes the job but terribly needs some UI improvements.

It doesn't work in some sites, like GMail and some of my office's servers (https://ibamanet.ibama.gov.br/docibama/). Also, it adds the [Show] link on the username field on GMail (no, it couldn't hide it, that would be really weird...)

Your addon works greatly on most sites but for popads.net, it breaks login fields just like in this pic https://screenshots.firefox.com/temUQeZS9Mvhi9G3/www.popads.net
Hope you get it fixed soon!