I tried to give Firefox and Mull a go again.
Mull hasn't changed.
Firefox Nightly now offers Tab strip (desktop-style tabbing) which is a mojor thing for me. It seems the scaling is now more or less OK when using "Desktop site" and manual font scaling. Drawbacks: The tabs squish when more tabs are added instead of sliding like in Opera. This is problematic with a large amount of tabs open. Also, there's still no option to permanently enable desktop sites. I have tried setting general.useragent.override, but that breaks the scaling again. It seems there have just been some improvements to "Desktop site" mode. The home tab also doesn't show up as a separate tab which is weird.
I wish they just kept the old mobile UI. There was even a separate tablet UI.