My utitlies website doesn't let you login if the password field is autofilled by the browser. Whatever Angular-based form validation they are using doesn't play nice with Firefox's saved password feature. You have to manually type something in the password field, so I always add and remove a space from the password.

I sent an email to their support, hoping they would fix it, but they just responded saying that they can't reproduce it.

Well, I can reproduce it. I even told you how. That sounds like a skill issue.