How would that work, as email masking is to protect online access? As I see it, Thunderbird is not in that workflow.

Just received info on Email Masking in Firefox.

If you're talking about Firefox Relay: when creating a new mask, every message sent to this mask is forwarded to the email account associated with your Firefox account. So all you need to do is to set up the email address associated with your Firefox account in Thunderbird (if not yet done already).