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.

Both are good

Librewolf uses Torbrowser configs, Mull uses the Torbrowser repo and entire config.

Torbrowser always uses the private browsing mode, which is really restrictive. Tabgroups do not work, cookies cannot be saved etc.

This makes MullvadBrowser way worse for daily browsing.

Torbrowser cannot use normal browsing mode, because they want to avoid saving data on the disk. Everything is in RAM.

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.

I use Firefox since all distros package it.

• 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.

Personally I like librewolf.

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.

Librewolf is Firefox on crack. Mullvad is Librewolf on crack.

Honestly at this point, normal firefox with ublock and custom user agent switched to chrome.

Just uses the one that works best for you.

They are about the same, ie good.

Wait. Mull browser is mullvad browser?

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.

Hardening aside, I like that LibreWolf actually lets you turn on JPEG XL.