Comment # 1 on bug 1131065 from Michael Pujos

This is not an openSUSE issue but a Thunderbird issue and new feature.
I agree this is annoying and I think it has been asked on the Mozilla
bugtracker for years to no avail.

Still the simple workaround to append .invalid to the "Server Name" of the
account you want to disable works:

imap.blargh.com => imap.blargh.com.invalid