I've had this issue for a long time, but always put it down to "I must have typed it wrong". It was undermining my sanity, but now I know I was always right! Sometimes when I try to log in to a website in Firefox, I will enter my password (correctly - some websites let you check what you've typed) and the website will return an error saying incorrect password. But, here's the thing... if I load the same website on Chrome or Edge and put the exact same password in, it will work. It will accept it. If it helps, most recently it happened on Visa Checkout pop-up for Ticketmaster. I didn't want to repeat the process in Edge as I would lose my tickets so instead I opted to reset password. I tried my old password and lo and behold, it said I cannot use my current password. Then... why didn't you let me in?

I don't know if it's a Firefox issue or the way the websites are coded (I'm not a web dev, I know nothing of how they are optimized for browsers), but is there any chance this sounds familiar and someone could investigate somehow?