LibreWolf

I assume you're specifically talking about Mullvad Browser, not just Mullvad in terms of, say, their VPN standalone.

I'd recommend you check out Privacy Tests for the specific details on things like tracking prevention, but to summarize, it's nearly identical to Librewolf in most common categories. However, since it's a fork of the Tor Browser, keep in mind that:

• There will be weird borders/spacing around sites, making them smaller than your browser, since that makes most Mullvad Browser users all appear to have an identical aspect ratio, which further prevents tracking, but can make sites a bit less usable.
• There's no built-in password manager, so you need to use your own.
• They don't support Firefox sync. (although unofficial bookmark syncing services do exist, such as Floccus)
• Some sites can look blurry because of some fingerprinting protection features
• Some third party extensions may fail to work properly, because they rely on the non-available Sync services.
• There's no mobile app.
• Your time zone is always spoofed to UTC so the way a site that depends on your current time could cause issues for you.
• Many fonts and hardware APIs are removed, so some sites that interact with peripherals may fail, and some sites may display incorrectly.
• All cookies are cleared between sessions. If you want to stay signed into sites without signing in every time you re-open your browser, you're out of luck.
• Some default extension functionality that normally comes bundled with Firefox is removed. (e.g. the screenshot tool)

If you're cool with getting a little extra protection in exchange for those sacrifices, go with Mullvad. If you just don't want to use Firefox, but want a more private variant you can still use relatively easily in everyday life, go with Librewolf.

And remember, it's always okay to have more than one browser, where you use the more privacy-preserving one for sensitive tasks when needed, then drop to the less privacy-preserving one for more everyday work. You can always have both.

It’s a bitch to use with 1Password. I didn’t experience downsides otherwise so far.

From a usability perspective it's not as good as Firefox. But from a privacy stand point it's much better.

What I mean about usability is that due to it trying to prevent you from being fingerprinted it opens the browser in the same size window everytime, regardless of whether you prefer maximised or not. It has dark mode turned off. It doesn't remember cookies unless you explicitly manually add an exception. From a privacy perspective these are all good things but for convenience they're not.

All of these mild inconveniences can however be turned off if you wish. Just be aware you won't be browsing as securely then though.

As a Firefox replacement in all other regards, it's pretty much the same software. No, it is the same software.

If you use Linux and a password manager you may have an issue getting flatpaks to speak to each other and you also may have to move a folder from .mozilla to .librewolf to get them to speak to each other. These are easily searchable issues if you have them with simple fixes though.

In all other regards, to me at least, it feels just like Firefox

As I understand it Librewolf is basically Firefox without Mozilla and almost all extensions I care about and use work perfectly.

I also believe you can import your Firefox configs into Librewolf without any hassle.

not everything is a smartphone app...

There's no mobile version, use Ironfox instead; they're both FF-based so you can sync between them.

Desktop only. Use this one for android IronFox (A privacy and security-oriented Firefox-based browser for Android.) https://f-droid.org/packages/org.ironfoxoss.ironfox/