Looks fine in portrait on my Samsung galaxy s20 on Firefox. Sure, it's not "mobile first class" websit, but it's a desktop application after all

Why? Were you planning to use it on mobile?

Looks fine in landscape on my hand-held.

Do you use your phone in landscape mode?

Yes, there's a neat button in a pull-down-menu where I can lock and unlock automatic switching based on some gyro-sensor or whatnot, it's kind of nice for when I'm watching Вампиры средней полосы or reading a technical manual.

I do when it makes sense to