I'm sure you've read about checking the system clock: certificate validity checks are affected by any error in date, time, or time zone, and sometimes allowing computers to use an internet-based time source can introduce errors. Assuming that isn't the problem... Does the untrusted connection error page show a code in parentheses (separated_by_underscore_characters)? Sometimes you need to expand a Technical Details section of the page to see the code. To provide a specific set of next steps, please let us know the error code (or codes) you are getting. ---- For example, among Mac users recently, a common code is '''sec_error_bad_signature''' and the most frequent reason for getting that seems to be the Avast Web Shield. Web Shield intercepts your browsing and filters it, but to filter secure sites, it presents "fake" certificates to Firefox. Avast should have set up Firefox to trust its fake certificates but that seems to fail on a regular basis. To test this theory, try turning off scanning of encrypted sites. I saw these steps in another post (hopefully they are applicable to Mac): #Open the Avast dashboard on the affected system. #Select Settings from the left sidebar menu. #Switch to Active Protection. #Click on Customize next to Web Shield. #Uncheck the "Enable HTTPS Scanning" option and click ok If that resolves the issue, but you prefer to filter encrypted communications, you may need to import the Avast! signing certficate into Firefox's Certificate Manager, Authorities tab.