It displays normally for me, using Mull, NoScript, UBlockOrigin, DarkBackground-light-text addon.
But for a moment it loads in the mode you described.
Do you have an addon that modifies your browser rendering? Like NoScript, UBlockOrigin, DarkReader, fingerprint protecting stuff, JShelter, or many others?
Sorry, read the text. Your Android is a version outdated, but thank Fairphone for that. They are notorious for shipping very delayed and incomplete updates...
Apart from that, we use the same version of Firefox (while I use Mull)