Heh, had a FF 1.5 bug happen a month ago. Turns out the user had auto updates enabled and THOUGHT he had the latest FF. He just didnt realize that it won't upgrade major versions automatically.
I had a friend who I was talking to and he was having trouble with Firefox. I asked him to check what version he was running, he told me 1.9.1. It was a minor miracle I realized that that was his Gecko version, because I was stuttering around like a crazy man, trying to understand how his browser was that out of date.