I checked it. I used the adblock.dns.mullvad.net option and adblocking works fine on all browsers without using adblock extensions. The checker on Mullvad’s website shows the DNS info as it should. I think maybe there’s no need for Firefox DNSoH settings anymore because the whole OS uses Mullvad DNS now. But I don’t know enough about DNS to be sure.

Is this also true about the cloudflare DNS over HTTPS option that Firefox provides in the privacy settings? If yes, then would it help if I changed the setting from ‘Cloudflare’ to ‘NextDNS’?

I use a file browser that also plays audio and video: Total Commander

It “remembers” recent playlist and play point of the file, as long as I don’t click the red ‘X’ in the controls.

It’s not on F-droid, but it’s possible to add its repository to F-droid.