This issue is not only being displayed in Firefox. I have faced it also in IE8.
It's pretty frustrating, since i'm using language detection in most of my modules, and they are currently throwing
A critical error has occurred. Input string was not in a correct format.
due to the fact that System.Threading.Thread.CurrentThread.CurrentCulture.ToString() or System.Globalization.CultureInfo.CurrentUICulture.Name.ToString() are both throwing Nulls when called for a different language than the default one.
When do we expect this issue to be fixed???
I propose a workaround for those who are facing my issue to use the following method:
Request.RawUrl.Contains("en-US")
this will test if your current url has the language code in it.
Sorry for the other posts, the page was hanging...