Mozilla is removing terminology from applications that has been identified as derogatory or exclusionary. We’re listening to conversations happening within the Mozilla community and in the world at large, and are paying attention when people tell us that some terms we use in our applications exclude and damage people.
‘Master-slave’ is a metaphor that perpetuates racism. Mozilla strives for inclusion and clarity; we have no need for terms derived from harmful metaphors when we have plenty of alternatives that are more inclusive, more descriptive and non-racist. For this reason, all instances of Master Password are being replaced with Primary Password in Thunderbird and other Mozilla products like Firefox.
Deprecating the term Master Password is also in accordance with the "Derogatory Language" section of the Mozilla Community Participation Guidelines.
This applies to Thunderbird 91 and above. You can find more information about the Primary Password feature here: Protect your Thunderbird passwords with a Primary Password