During the summer of 2017, we switched to a new internet filter called Securly. This new filter is web based and uses a certificate in order to function.
Please follow these steps:
If you haven't already, download the certificate above.
Open Firefox, click Firefox in the top left (next to the Apple logo)
Click Preferences, then Advanced
Click the Certificates tab, then click View Certificates
Click the Authorities tab, then click Import
A window will appear, navigate to the Downloads folder, then double click on the securly_SHA-256.crt file
Another window will appear, be sure to select the top option, "Trust this CA to identify websites", then click OK.
Either refresh the page in Firefox or quit it and reopen it and attempt to access your gmail.