Master/servant implies different levels of authority and control.
Personally - I find the later much more compelling from a descriptive standpoint for this use of passwords.
Honestly, there are a ton of alternatives that I think make more sense - ex - Control password - Premier password - God password (although this probably violates some other policy...)
etc...
but primary is more familiar and and they probably liked the alliteration. Really - this just feels like marketing, though.
It's a reason to make people think & talk about Mozilla that's.... drumroll... once again not actually a compelling product or experience.
It's so frustrating that an issue inside American society impacts 93% of the World who is not American.
We should drop American english as primary language and in this case use the German meister or even better the Italian mastro or its Latin root magister
Which are all translations of master and have no relation with slavery of any kind.
Like we also did to disambiguate free (gratis - another Latin word ) from libre (from Latin - again - liber)
English is too ambiguous and irregular.
EDIT: not for these decisions, but incidentally using Libre in the name, my goto alternative has become Librewolf. For the few websites that are broken in LW I use Brave.
A chess grandmaster does not imply the existence of chess grandslaves. The Sega Master System didn't enslave other game systems. A master key is just a key with wider scope than the regular keys.