User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:80.0) Gecko/20100101 Firefox/80.0

Steps to reproduce:

I have 2 entities on the same account (2 addresses : one is alias of other) : screen-capture -1-
I have 2 S/Mime certificate, for each address of the entities (the mailbox and the alias) I have a valid S/MIME certificate imported. Screencapture -4-

Actual results:

When I try to compose a mail with the first identity (the real mailbox) the certificate is automatically selected, and the interface proposes to sign by default. : screen capture -2-
But when I select the second certificate, the interface says it is not possible to sign. screen-capture -3-

Expected results:

I would have expected that if I select the second identity, the interface automatically selects the corresponding certificate.

As signing is tied to the email address, and different identities can easily have different email addresses (the main use case I guess), you have to go into the second identity and specify what certificate you want to use for that.