Following the pro-Mull comments here I’ve given it a try for a solid 48h, and just reverted back to Fennec. Mull is simply restricting the user experience too much, and I’m not willing to make the sacrifice.
My biggest annoyances:
Websites don’t get information about dark mode from my device and revert back to light mode by default.
Websites don’t get information about the system time on my phone and deliver content based on GMT+0.
Some websites get wrong (or none?) information about the screen resolution and are unusable.
I’m aware that those details are suppressed to avoid fingerprinting, and while I believe that the intention is good, it makes using my phone more cumbersome, and that’s not something I’m willing to do. So my choices at this point are basically to keep using Mull and deactivate the advanced fingerprinting protection, or use Fennec as before.