It's basically the same as LibreWolf with "resist fingerprinting"/letterboxing enabled, strict mode toggled, and NoScript and uBlock Origin pre-installed.

I had/have big hopes for Mullvad Browser because browser anonymity increases with the size of the crowd using it.

Having those defaults means everyone using Mullvad Browser has the same default extensions, much like Tor. One doesn't "stand out" as much as manually installing them, thereby increasing your unique browser fingerprint.

"it is imperative that you do not modify the browser at all outside adjusting the default security levels. Other modifications would make your fingerprint unique, defeating the purpose of using this browser. If you want to configure your browser more heavily and fingerprinting is not a concern for you, we recommend Firefox instead."

-- (privacyguides.org)

I keep my browser extensions down to the minimum, but I do have others.

This makes you stand out more using Mullvad Browser, so does not using their VPN. I use Mullvad VPN as a backup, but I prefer ProtonVPN, purchased over Tor with non-KYC BTC, with no recovery email or phone set, separate from my main Proton account. Because UDP only WireGuard gets blocked by too many sites, streaming platforms etc.

**I use and recommend Mullvad for specific tasks, but due to these limitations, it still hasn't replaced LibreWolf for me.**

Here's more info on OpenVPN and WireGuard: