The Mullvad Browser is based on the Tor browser, but it doesn't use the Tor network, whereas LibreWolf is based on Firefox + arkenfox user.js. LibreWolf is better for normal day-to-day browsing, where as Mullvad is meant to be used for high privacy/security tasks. Mullvad is kinda hard to daily drive, because it can't be configured to save cookies, you can't really use extensions and it lacks some other things. These features were removed in the Tor browser, because as I said, it's meant for high thread model usage. Edit: I like the Mullvad browser and I use it myself, but not as my daily driver.