LogIn not viable for Firefox 94.0.1 (64-Bit)

None of the other 50% of the users on Win10 Firefox are having this issue, so I reckon you need to look quite carefully at how your Firefox browser is cacheing first of all user sessions and second of all cookies (which I know you already said you cleared, but remember how aggressive browsers are these days with cacheing and storing things for you). I changed the cookie name this patch which is what would typically lead to an error like this - so focus on cookies related stuff. Nothing I can do my end and hope the clue will lead you to getting it fixed in FF. A computer restart sounds stupid but could actually reset some authentication/cookies stuff in FF's browser - worth a shot.