I assume you’re specifically talking about Mullvad Browser, not just Mullvad in terms of, say, their VPN standalone.

I’d recommend you check out Privacy Tests for the specific details on things like tracking prevention, but to summarize, it’s nearly identical to Librewolf in most common categories. However, since it’s a fork of the Tor Browser, keep in mind that:

• There will be weird borders/spacing around sites, making them smaller than your browser, since that makes most Mullvad Browser users all appear to have an identical aspect ratio, which further prevents tracking, but can make sites a bit less usable.
• There’s no built-in password manager, so you need to use your own.
• They don’t support Firefox sync. (although unofficial bookmark syncing services do exist, such as Floccus)
• Some sites can look blurry because of some fingerprinting protection features
• Some third party extensions may fail to work properly, because they rely on the non-available Sync services.
• There’s no mobile app.
• Your time zone is always spoofed to UTC so the way a site that depends on your current time could cause issues for you.
• Many fonts and hardware APIs are removed, so some sites that interact with peripherals may fail, and some sites may display incorrectly.
• All cookies are cleared between sessions. If you want to stay signed into sites without signing in every time you re-open your browser, you’re out of luck.
• Some default extension functionality that normally comes bundled with Firefox is removed. (e.g. the screenshot tool)

If you’re cool with getting a little extra protection in exchange for those sacrifices, go with Mullvad. If you just don’t want to use Firefox, but want a more private variant you can still use relatively easily in everyday life, go with Librewolf.

And remember, it’s always okay to have more than one browser, where you use the more privacy-preserving one for sensitive tasks when needed, then drop to the less privacy-preserving one for more everyday work. You can always have both.