Currently, I don’t use a VPN.
It states in the PrivacyGuides resources that fingerprinting protections of Mullvad Browser are far less effective than with a VPN. Since CGNAT is where many people share the same IP address, how is using Mullvad Browser in this case? How comparable is this to a custom hardened Firefox?
It depends on how many people are sharing that IP address (and how many of those people are using Mullvad Browser).
The basic idea is to be part of as large of an anonymity set as possible. Using Tor Browser with the Tor network basically guarantees you’re part of a fairly large group of users. Using Mullvad Browser with a VPN that is highly likely to have other Mullvad Browser users (such as uh… Mullvad lol) gives you similar assurances.
Using Mullvad Browser with other VPNs or other shared networks can provide the same benefit, but it isn’t as much of a guarantee.