Firefox uses in memory caching for JavaScript and can cause strange errors in your MSFT ASP.NET Ajax applications.
Try adding this to your load event to prevent those errors:
if (Request.Browser.MSDomVersion.Major == 0) // Non IE Browser?)
{
Response.Cache.SetNoStore(); // No client side cashing for non IE browsers
}
{
Response.Cache.SetNoStore(); // No client side cashing for non IE browsers
}