In Firefox Browser When I Click On Textbox Field The Page Move Upward

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C#

HTML

Firefox

I have a customer registration page
the issue is that there is a textbox with some javascript method which gets fired o typing number in the textbox
There is no issue all working fine in Internet explorer but the problem arisses in firefox
When i click on that text box the page moves upside and i am unable to type anything to that Textbox the cursor is on the textbox only but on typing anything nothing happens, cant write anything on that whereas same is working fine in internet explorer.

<asp:TextBox ID="txtRegSearchPincode" CssClass="textboxostyle" Text="Search By Pin"
                                                                               ToolTip="Search By Pin Code" Width="80px" MaxLength="6" onkeypress="return AllowNumericRegPinCodeOnly(this,event);"
                                                                               runat="server" onblur="WaterMarkSearchPinCode(this, event);" onfocus="WaterMarkSearchPinCode(this, event);"></asp:TextBox>

function WaterMarkSearchPinCode(txtPincodeSearch, event) {
       var defaultSearchPinCode = "Search By Pin";

       // Condition to check textbox length and event type
       if (txtPincodeSearch.value.length == 0 & event.type == "blur") {
           //if condition true then setting text color and default text in textbox
           txtPincodeSearch.value = defaultSearchPinCode;
       }
       // Condition to check textbox value and event type
       if (txtPincodeSearch.value == defaultSearchPinCode & event.type == "focus") {
           txtPincodeSearch.value = "";
       }
   }

Why is this happening ??