I think this could be the right forum to inform the Xamarin people about some malfunctioning of the website/forum on Firefox browser.
I could not register so I had to install Chrome, and I was successful. Now I can sign in only with Chrome.
I hope this is useful. I did not tested the other browsers.
Furthermore I got some server errors also when posting in Chrome on this forums: submitting the question the first time does nothing, then I save draft but it can give an error, and so on.