Could you
1. go to about:preferences#privacy (either by typing/clicking in the link it or through menu Options and
2. click on Privacy & Security
3. scroll down to Certificates
4. click on View Certificates.
5. Select Avast Web/Mail Shield Root - Software Security Device
6. click Edit Trust.
The checkbox This certificate can identify websites should be checked. If not, please do so.
Does it help?
(we are working on a fix, but it would help us to confirm that this workaround worked for the case when Firefox shows 'Your connection is not secure')
Sadly mine was already checked so there was nothing to do there.
Also, my partner has Bitdefender 2019 and the same error is happening with Firefox on that computer too (only working when you deactivate the HTTPS equivalent or change the Firefox setting). It looks like Mozilla really jumped the gun with this update.
I reverted my original workaround because I don't think it's safe to have no HTTP scanning, and changed the Firefox setting instead, although I echo the other user that it asking for the Master Password each run is annoying. I just wish I could use the quick-fixes that seem to work for many others! But no dice, for whatever reason.