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.

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.

Both are good

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.

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.

• 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.

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

Personally I like librewolf.

Just uses the one that works best for you.

They are about the same, ie good.

I've had Mullvad installed for around a year or more. I turn to it from time to time when I wanna keep things separate from my regular browser, like if I'm looking into items on Amazon that I only need once and don't want recommendations to get polluted. For example, I was looking at the price of spinning platter HDs after one failed in a NAS. I don't want Amazon trying to sell me more old-tech drives once I replace it.

Has worked well so far. Haven't tried the other one.

Wait. Mull browser is mullvad browser?

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

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.