thanks for reporting the problem, so far we have heard about this from avast/avg users whose security software is meddling with encrypted connections. firefox 61 introduced the better security protocol tls 1.3 & apparently avast can't cope with that yet.

as a workaround, please try to disable the scanning of https connections as described in https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message#w_avast-and-avg-security-products

Philipp:

Yes, I should have mentioned that I am using the latest free version of Avast.

I am not going to tamper with any of the workarounds that you suggested. Instead, I posted this issue in the Avast forum so I will wait for them to offer a solution.