Im running firefox 58.0b4 developer edition due to a problem when my old firefox updated and refused to run legacy apps. So its been running fine since I started using it till yesterday when I keep getting the Your connection is not secure notification but only on a small number of pages that I know have good certificates. Also when I fire up my other version of firefox, I dont have this problem. Date and time settings are fine and nothing else have changed as far as I know.