

Well, the hardening, just as with Tor Browser, does break some sites. It comes preinstalled with NoScript and uBlock Origin, the former of which you will either have to learn how to use or disable, depending on your wants for privacy. While it doesn’t include some of the anti-features of base Firefox, it is still based on Firefox so it will have similar performance for similar tasks.
Personally, I use Mullvad for most of my browsing, and Firefox for a few specific things (like staying logged into site long-term and such).
It’s available as a flatpak via Flathub for an easy installation, otherwise you can check https://mullvad.net/en/browser/linux for distro-specific installation instructions.
Yes, it is possible, as per your linked resource, however it would be up to the server maintainers if they feel it is reasonable to do. Note that federation is not supported over Tor, so federation would go over the clearnet via the original instance. This is not an issue, because the only thing that would be useful for Tor is hiding the client connection IP address, which would be handled under this configuration.