LibreWolf has an official Flatpak, which is great for Linux users. The Mullvad Browser Flatpak is not official and allegedly makes suspicious connections.
LibreWolf has good private default settings, but the Mullvad Browser and Tor Browser can go the extra mile because they're meant to blend into a crowd, through VPN traffic and the Tor Network respectively. If you plan to log-in to sites, install extensions, enable uBlock Origin lists, or customize your browser at all, you defeat the purpose of those browsers.
LibreWolf by comparison is far more reasonable to customize. They even have a custom settings page for the more intense privacy settings among other things. Enabling persistent storage for sites you log into is easier because it's not forced into Private Browsing all the time. These changes are reasonable because the goal is not to blend into a specific crowd.
There's no reason you can't use all these browsers (and more) if their use cases sound relevant to you. I use LibreWolf, Mullvad, Tor, Brave, Vanadium, Mull, and even Edge depending on my needs.
As the other guy said, its basically firefox but with really strong defaults out of the box. Its a good easy solution for the majority of people who want a browser that gets the job done.
I install it, get a dark mode extension, maybe change the search engine if I dont like it (Default was DuckDuckGo, but I believe that changed) and im ready to start browsing. Deleting all cookies and history every time I close it is also very nice. (Though you can make exceptions if you like)
Its the best browser for privacy that doesn't use tor. It has the capability of fooling naive scripts but also may have some protection against stronger fingerprinting methods since you blend in with a crowd.
Personally I prefer arkenfox because I dislike letterboxing, want sync, want to have control over my user overrides. Disabling these things in mullvad will likely make me stand out. IMO if your threat model is that high I would just use tor and a hardened FF or brave for normal use. All individual preferences though.
Its a modification to an official install of firefox. In firefox you can go into about:config and modify each setting individually. All arkenfox does is set everything for you with a script and you can make an overrides file to tweak their config. See here
For people who don't want to tweak I generally recommend librewolf. Its a forked firefox client that has quality of life features and ability to change certain settings within your browser's settings tab. The only downside of using librewolf is updates can fall behind, which can be a security issue. Their team has been on top of updating though.
For my everyday surfing without needing an account, I'm using the MB. For everything else It's LibreWolf. I use Chocolatey to get updates so I don't fall behind. Winget is also possible.