Thunderbird opens correctly only 10% of the time

There can be a few reasons for certificate errors like that, including: - You could be using incorrect server settings in Thunderbird. You should verify them by checking on the provider's help page or contacting the provider's tech support. - Anti-virus, network security, and VPN programs are notorious for co-opting normal authentication processes. If you use any of those, as a test, try temporarily disabling them or creating an exception for Thunderbird. - They could, indeed, be having server and/or certificate issues. They may already be aware of it and be working on a fix, so if you were to wait a while it may go away when they do, or you can contact them for status.