Tab key is not working in javacript in firefox MVC

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Javascript

jQuery

validation

Tab

, +

In Firefox, If I Tab on the controls in MVC View, then tab key is not working. it doesn't go to the next control.

Quote:

@Html.TextBoxFor(model => model.FirstName, new { @class = "form-control", @placeholder = "First Name", @id = "txtFirstName", @maxlength = "50", onkeypress = "return onlyAlphabets(event,this);" })
@Html.TextBoxFor(model => model.LastName, new { @class = "form-control", @placeholder = "Last Name", @id = "txtLastName", @maxlength = "50", onkeypress = "return onlyAlphabets(event,this);" })

function onlyAlphabets(e, t) {
        try {
            if (window.event) {
                var charCode = window.event.keyCode;
            }
            else if (e) {
                var charCode = e.which;
            }
            else { return true; }
            if (charCode == 9 || (charCode > 64 && charCode < 91) || (charCode > 96 && charCode < 123))
                return true;
            else
                return false;
        }
        catch (err) {
            alert(err.Description);
        }
    }

Thanks

What I have tried:

i have tried with

charCode == 9 

allowing in js code.