Apparently Firefox now defaults to picking website light/dark-mode based on... the color theme you've picked for Firefox UI (as if anyone ever changes that). The default one, despite being quite dark, is considered "light".
See screenshot attached.
Short-term solution: select "Dark in settings.
Long-term solution: software vendors stop doing stupid shit and overriding user preferences like that.
Correct default should be "System theme" - that's literally what it's there for!