This issue can be caused by corrupted cookies.

Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Firefox > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Firefox > Preferences > Privacy > Cookies: "Show Cookies"

This is what worked for me on a Mac, go to Preferences then Advanced, under Connections click Settings then select Auto-detect proxy settings for this network. After I did that, I Got Mail! Original setting was: Use system proxy settings