It looks like they might use Mullvad's DNS Over HTTPS by default in the Mullvad browser and this would probably be the biggest privacy thing, but whatever your default DNS is might be a larger privacy thing. Your ISP or Google's 8.8.8.8 traveling unencrypted is probably a bigger issue.
It looks like Mullvad is also based off the Firefox ESR (extended support release) version that the Tor Browser uses while LibreWolf would be more up-to-date: https://news.ycombinator.com/item?id=35421718
It looks like they might use Mullvad's DNS Over HTTPS by default in the Mullvad browser and this would probably be the biggest privacy thing, but whatever your default DNS is might be a larger privacy thing. Your ISP or Google's 8.8.8.8 traveling unencrypted is probably a bigger issue.
It looks like Mullvad is also based off the Firefox ESR (extended support release) version that the Tor Browser uses while LibreWolf would be more up-to-date: https://news.ycombinator.com/item?id=35421718