Two common reasons for this are:

(1) Problem with the system clock: date, time, or time zone differs substantially from the rest of the internet

(2) Firefox not set up to work with your security software that filters your web traffic

In order to filter secure traffic, your security software needs to present "fake" certificates to Firefox so it can be the "man in the middle" and see everything sent to and from the site. Firefox distrusts the fake certificates until instructed otherwise.

Do you use any software with that kind of feature? On Windows, products with that feature include avast! 2015, Bitdefender, ESET, and Kaspersky.

A less common problem is malware performing a "man in the middle" attack.

Thank you very much.

Turns out it was Avast Security on my Mac, once disabled Firefox worked flawlessly. Had to uninstall both, and reinstall Firefox first, Avast second in order for Firefox to accept the Avast cert.