Mozilla VPN is a wrapper for Mullvad VPN.
There may be some separation of identity advantage if Mozilla holds your personal info instead of Mullvad (I can only guess if that’s the case) but the choice would probably be which software between the two works best for you.
More info:
EFF brand agnostic recommendations: Choosing the VPN That's Right for You | Surveillance Self-Defense
Amazing comparison site (incl. Mullvad): https://thatoneprivacysite.net
Decent summary of leading brands (incl. Mullvad): Best VPN for Privacy & Security in 2023 - Top 5 - Best VPN Services - privacytools.io
DLN Community run downs: