Confirming what ekjhgkejhgk is saying.

It really just is Firefox with more privacy tuning. As far as I'm aware all the add-ons work as expected. I've used it as a trial and can confirm ublock works perfectly fine but that's the only add-on I tested.

Also, it is bundled with a mullvad add-on, but it is easy to remove.

You should also go to the Privacy and Security tab in the browser. By default it is set to Max Protection with Mullvad DNS by default. Even their lowest security is better than Firefox. But I would suggest editing this the "Mullvad (Ad-blocking)" option. I believe this is the same DNS as adblock.dns.mullvad.net (194.242.2.3)[0], which (base.dns.mullvad.net (194.242.2.4) is better) will be pretty similar to PiHole style ad-blocking.

I haven't tested in browser (I did test when setting up my PiHole) but Mullvad DNS can be a little slower compared to quad9 or cloudflare. But I don't think those two have ad blocking (and DNS ad blocking can be better in a lot of ways because it is not being blocked user side)

Btw, you can do this DNS stuff in vanilla Firefox too.

[0] https://mullvad.net/en/help/dns-over-https-and-dns-over-tls