moarTLS od EricLaw

HTTPS only works if you use it! This simple extension adds a button to Firefox that allows you to easily discover non-secure (e.g. HTTP) links on your pages.

Simply click the moarTLS button and non-secure link references in the current page are flagged for review.

You can learn more about latent mixed-content vulnerabilities here: https://textplain.wordpress.com/2016/03/17/seek-and-destroy-non-secure-references-using-the-moartls-analyzer/