You can compare Mullvad Browser with your Firefox Browser. Just do a Fingerprint Test:
coveryourtracks.eff.org
Is your Firefox better? Does it hide more details about you? No.
Your Firefox is likely very unique. So your settings and extensions appear to be special; let’s say you stick out of the others. Also your Screen-Size and if its a touchable screen and how much RAM your device has and so on… Mullvad Browser makes all users look much more like the same user - users are less identifiable.