Arrow keys not working in firefox with given javascript

I have one textbox coded like below :

<input name="txttelephone" type="text" maxlength="16" id="txttelephone" class="InputTextBox" onkeypress="return NumbersOnly(event);" required />

And javascript function is like below :

function NumbersOnly(e) {
    var unicode = e.charCode ? e.charCode : e.keyCode;
    if (unicode != 8) {
        if (unicode < 48 || unicode > 57) {

            if (unicode == 9)
                return true;
            else
                return false;
        }
    }
}

Now When I run this in chrome arrow keys working proper but in firefox arrow key is not working. Not getting what is the issue.

Please help me with this.

Thanks,

Dipa

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