The below is in part thinking out loud, not intended to be an authoritative/definitive answer
I don’t know. And I’m not sure that it is knowable because I don’t think we can accurately estimate Mullvad Browsers userbase.
I do think that (for desktop) it is your best chance. A browser that ‘enforces’ uniformity to a high degree like MB or TBB, should be able to get by with a smaller userbase, compared to a browser that does not.
From my understanding from reading on the Mullvad website, you have to have people be on the same VPN connections for that pool of unique users to work countering fingerprinting.
Protecting your IP (with a reputable VPN) is indeed important (regardless of whether or not you use Mullvad Browser). And its even more important if you are going through all the trouble of defending against browser-fingerprinting.
But I’m not sure that that Mullvad Article says precisely the same thing that you stated (“you have to have people be on the same VPN connections”). While it is definitely best if a ton of Mullvad Browser users are concurrently using the same handful of VPN servers, I think that Mullvad Browser can still have utility even if you are maybe using a reputable but less popular VPN.
Consider from the perspective of an adversary. seeking to identify individuals (not targeting you personally, just opportunistic surveillance capitalism), how will this adversary determine whether all the connections they see from the VPN servers IP, are a single person’s repeated visits, a few people visiting a few times, or many people visiting once or twice? (assuming that MB does a good enough job protecting against fingerprinting and traditional tracking methods). And assuming you don’t always connect to the same VPN server, how do they reliably link your browsing across sessions? I wouldn’t bet my life on that, but I would trust it as an approach that is probably effective against untargeted and opportunistic/passive tracking and the low hanging fruit. And for more critical scenarios, I’d move to Tor Browser or Tails.
For the best chance of blending in, I’d imagine you’d want to be using Mullvad Browser + Mullvad VPN is the way to go. But I still see value in Mullvad Browser + another VPN (particularly one of the other PG recommended ones).
I’m curious to hear what others think on this topic, or if there are flaws in my thinking here?