document.getElementById(id).focus() is not working for firefox or chrome

Try using a timer:

const id = "mobileno";
const element = document.getElementById(id);
if (element.value.length >= 10) {
    alert("Mobile Number Should be in 10 digits only");
    element.value = "";
    window.setTimeout(() => element.focus(), 0);
    return false;
}

A timer with a count of 0 will run when the thread becomes idle. If that doesn’t help, try the code (with the timer) in the onblur event instead.