FireFox have problemes for getBBox(), i need to do this in vanillaJS.
I've a better Way and is the same result as real svg.getBBox() function !
With this good post : Get the real size of a SVG/G element
var el = document.getElementById("yourElement"); // or other selector like querySelector()
var rect = el.getBoundingClientRect(); // get the bounding rectangle
console.log( rect.width );
console.log( rect.height);