Password field with numbers only

I'm creating a web app and i'd like to have a special password field for smart phones.

Users passwords will be only numbers so I want a special keyboard (It's done and it works)

The problem is Firefox for android, the password field is not displayed as password but as simple text.

My ASP.NET field :

 <asp:TextBox Type="Number" pattern="[0-9]*" CssClass="PassTxtBox" ID="Txt_psswd" runat="server" Columns="20" MaxLength="20" Width="100%" Height="30px" ForeColor="Black"></asp:TextBox>

My CSS :

.PassTxtBox{
-webkit-text-security: circle;
}