Is Mullvad better than Firefox?

That's hard to qualify. The question needs to be more specific. Is Mullvad better than Firefox* at what*? Firefox is a great general browser with decent security and privacy in mind. It allows you theme and modify to your desire. However, any time you add theming or extensions it makes your browser more unique and identifiable. The more you add, the more unique. Stock Firefox is a little promiscuous for my liking and I usually install UBlock Origin and add a little css, like Betterfox. You can create a new Firefox profile which you can swap between depending on the purpose of the window. Or, you can just add a user.js file to your existing Firefox profile. This is called "hardening", and there's many different hardening css available. Some make most things unusable, so a balance is recommended. If you're on Linux you could just use Librewolf which is a sandboxed Flatpak app that is built from Firefox, and, has a great balance of security and privacy tweaks out-of-the-box. Then we have Mullvad browser. Is it better? Maybe it isn't as fast. Maybe it doesn't open some web pages that stock Firefox would. However, Mullvad is brilliant at making you blend in. Mullvad have created the browser with a great balance of privacy and security tweaks that harden it somewhat. What it does, just like Tor browser, is make your online "fingerprint" look like thousands of other people's browsers. As long as you don't identify yourself somehow there's a better chance at anonymity. Identifying yourself could include logging in a known account, adding themes or extensions or using social media. Read more about it here. I recommend you use both. A lightly hardened Firefox that you use for general purpose, and a Mullvad for browsing, searching and shopping (not purchasing). Mullvad browser is best used with a VPN that lumps your IP in with many other VPN users, like MullvadVPN or IVPN.

I hope this helps.