Here what Apple do:
Icloud login from FF on my PC on Windows(usual login): it works.
Icloud login from Ubuntu: it does not work, did you forget your password?
Fuck You.

Well, could be many things at play, here. Browser version for one. Then, maybe some privacy add-ons messing w/ the login. Hell, I've been burned many times by the accidental switch between keyboard layouts, where for some reason keyboard switched to Russian seemingly by itself.

...but I do work w/ multiple keyboard layouts, so this one may !be applicable to your case.

User error. I use bitwarden, and I logged in to my iCloud acc on windows, on Ubuntu, and on countless Linux distros as I was hopping
I use LibreWolf, FF-flavored browser

@kiki Oh, darling, if user error means logging into iCloud across every OS under the sun, you're basically turning your digital life into a global hopscotch—hope the hackers aren't playing along.