Both are good.
Librewolf is more like vanilla Firefox, just configured way better by default.
Mullvad Browser is like a port of the Tor Browser (also based on Firefox) for the clear web (or for use with Mullvad's VPN, or whatever). Also configured very well by default.
Mullvad Browser has better anti fingerprinting stuff built-in but as a result of its unusual configuration some sites might be broken.
Librewolf is kind of the opposite in that regard - sites won't be broken but you'll be easier to fingerprint.
In any case, they both are at the top of the best Firefox variants I'd say.
The "opposite" was just referring to those 2 aspects - Mullvad has stronger anti-fingerprinting which leads to more breakage. Librewolf has that aspect reversed. Of course, both browsers are similar overall. That's just one detail where they prioritize differently.
I like LibreWolf, but I don't like that it wipes cookies and session tokens each time you launch it. I understand why they do it, but it's a consideration outside my threat model, so it just annoys me.
You can also easily set specific sites' cookies not to be wiped, I use this to have websites I trust to store my data for convenience, but any random tracker-infested blog to forget me as soon as the browser closes!
And also, Mullvad Browser does this by default, as well. I think theirs can't even be configured on a per-site basis.
Librewolf comes packaged by my distro (GNU Guix) so that's what I use. I'm sure most "privacy" or "hardened" Firefoxes are more or less interchangeable. The only one that's really noteworthy is GNU IceCat, because it's more focused on software-freedom and includes the LibreJS addon, but I switched to Librewolf once it was packaged for Guix.
trust: The biggest trust factor difference to me is, who manages the package and how it is installed. Both are not packaged by my distribution maintainers, therefore I have trust issues with a program that important. However both are available as Flatpak. So I would recommend to install it this way.
updates: Another big factor is how often these are updated, especially security patches. In example for any Firefox based browser, I would not want to wait longer than 1 day before the fork is on the same version as the mainline Firefox.
I personally would prefer LibreWolf over Mullvad, because it is based off Firefox.
Firefox + Arkenfox ... :P
Using Mullvad as a backup before having to use chromium for any dirty stuff. Librewolf seems to be like a good recommendation for non-technical people. Although I am not sure which browser has less breakage Mullvad or Libre.
No, mull is a fork of Firefox meant for android and developed by the divestOS team, while mullvad is a completely different fork, only for desktop atm and developed by mullvad (the VPN company)
they both focus on privacy and integrate Tor patches and modify some configs to better resist fingerprinting, but mullvad browser goes a step further with the tor integration, going as far as storing all data in RAM, so it deletes every cookie, history etc on restart. Could be an advantage, could be a disadvantage, up to you
IIRC it might be on by default (tho this would hurt anonymity if you can request JXL files & stock Fx cannot), else open about:config & search for “jxl”. Upstream has kept this flag—toggle on or off—only working in Fx Nightly.