User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:87.0) Gecko/20100101 Firefox/87.0

Steps to reproduce:

thunderbird 78.7.1 (64bit) on GNU/Linux Ubuntu 20.04 LTS platform.
I learned the certificate check with wireshark analyzer on my working system.
found the TCPport 80 HTTP-OCSP transaction for the verification of the certificate. Now I got curious and blocked the OCSP server IPv4 address with my firewall (ntf in Ubuntu 20.04 LTS) and reset the timeouts down to 16 sec in the Linux kernel.

Actual results:

I know that the systemcall connect() must return now with ETIMEDOUT.
for the 3rd party certificate check and expected a popup in thunderbird
like "3rd party connect for certificate verification failed with a timeout,
can not access the mailbox" (by the way the config is IMAPS)
but really nothing reported. No hints, the mailbox looks empty.

Expected results:

a clear hint for the root cause
"3rd party certificate verification failed with a connect timeout"

found in thunderbird->about:config security.ocsp.require and changed it from FALSE to TRUE
this brought me closer to my "hard security" mode, but still no clear information about the root cause.
the lower status line shows activity and stops after about 16 sec.

I did the same with firefox "about:config security.ocsp.require TRUE"
and here is a much better message pointing to OCSP