backspace isn't working in number filed of html5 contact form in firefox

I am trying to fix some javascript error of contact form while user trying to enter the contact number they cant able to correct or cant able use backspace key in firefox only. In chrome it is working completely fine

here is my code

CG = (function(){
    var
    _isNumeric = function(evt){
        evt = (evt) ? evt : window.event;
        var charCode = (evt.which) ? evt.which : evt.keyCode;
        if(charCode >= 48 && charCode <= 57)
            return true;
        return false;
    },
    _isDecimal = function(evt){
        evt = (evt) ? evt : window.event;
        var charCode = (evt.which) ? evt.which : evt.keyCode;
        if((charCode >= 48 && charCode <= 57) || charCode==46)
            return true;
        return false;
    },
    _alphanumeric = function(evt){
        evt = (evt) ? evt : window.event;
        var charCode = (evt.which) ? evt.which : evt.keyCode;
        return ((charCode > 64 && charCode < 91) || (charCode > 96 && charCode < 123) || charCode == 8 || charCode == 32 || (charCode >= 48 && charCode <= 57));
    };
    return{
        isDecimal:_isDecimal,
        isNumeric:_isNumeric
    }
});

    $(document).ready(function(){
    $("input[type=text]").on("keypress",".isDecimal",function(e){
        return CG.isDecimal(e);
    });
    $(document).on("keypress",".isNumeric",function(e){
        return CG.isNumeric(e);
    });
    $(document).on("keypress",".alphanumeric",function(e){
        return CG.alphanumeric(e);
    });
    $(document).on("keypress",".isDecimal",function(e){
        return CG.isDecimal(e);
    });
})