The thing here is that users reporting this can enter into some pages but not on, at least, a particular one. Cookies are for the whole site and I don't think that particular pages is sending a particular cookie. With firewall the story is more or less the same, because it's very unlikely that it will treat differently one page from another being of the same site. My bet is there is something on that page that perhaps exposes some Firefox bug, probably a js one, but without the console log is quite hard to know. It complains about a redirection, so a look at the network section of the inspector would be helpful too. At least is what I would ask if I was the developer. That said, in the world of bugs everything is possible.
Without more evidence from the problematic side, we can be here all night speculating, which isn't necessarily a bad thing if we are in good company.
I use Firefox but in Ubuntu, so not comparable. I've been looking at the problematic page but I don't see anything special. Just by chance I've detected a minor bug when an emoticon is inserted, but that doesn't cause any other problem. But this investigation procedure is bound to fail. The only one hope is for someone with the problem to send more information: console log and network page.
