It's because the thunderbird is built against nss-3.44 but our recent RHEL8 nss is 3.41. You need to upgrade to nss-3.44 or rebuild TB locally with nss 3.41.
We're going to release next security update - thunderbird-60.8.0 - so we can fix this bug there.
*** Bug 1727327 has been marked as a duplicate of this bug. ***
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.