Button trigger UpdatePanel fails in FireFox

Firefox drove me crazy! I have spend hours to work out why it does not work, and the reason is: it does not have an image url but have alt text (and my Button is an ImageButton).

To demonstrate this, here is an example:

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <div id="Html" runat="server" Visible="true">
        Please submit
        </div>

        <asp:ImageButton id="SubmitButton" ImageUrl="submit_btn.gif"
 onclick="SubmitButton_Click" alt="Submit" runat="server"
 OnClientClick="this.style.display='none';"></asp:ImageButton>
        <asp:ImageButton id="AmendButton" onclick="AmendButton_Click" alt="Amend"
 runat="server" OnClientClick="this.style.display='none';"></asp:ImageButton>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="AmendButton" EventName="Click" />
        <asp:AsyncPostBackTrigger ControlID="SubmitButton" EventName="Click" />
    </Triggers>
</asp:UpdatePanel>

And the code behind is very simple:

        protected void SubmitButton_Click(object sender, ImageClickEventArgs e)
        {
            Html.InnerHtml = "Please Amend";
        }

        protected void AmendButton_Click(object sender, ImageClickEventArgs e)
        {
            Html.InnerHtml = "Please Submit";
        }

The page looks like this:

If you click the submit button, it becomes:

As expected. However if you click the alt text "Amend", it will refresh the whole page instead of updating just the update panel. Basically I mean it does not work. The only solution is, of coz, give it a valid image!!!

Wasted me several hours! Now I am going home, it's 7:00pm already!