I had an ex do the same thing. It was this site she saw messages from, actually. She went on my computer one day (I'm assuming out of insecurity, which it seems is a growing problem in women), and went through my history, I have my login for FF saved, and she read through all the little complimentary messages I had sent to girls on the site about their pictures or videos or what-have-you and accused me of "cheating". She claimed she could "no longer trust me" and for months I had to make sure that if I visited FF at all (which was nearly impossible as she watched me like a hawk) I had to delete my internet history every time. It was aggravating and made me miserable, and we eventually broke up, fighting horribly via verbal comments. I'd say that this girl you are speaking of was definitely in the wrong and her freaking out over something like that is definitely her own problem, since she invaded your privacy.
14 years