Figuring out sessionStorage size limit of a browser

I was able to figure out the size of sessionStorage that is used by my app using the following line of code in console. I used the following code from the first answer of this stack overflow post I just had to change it from localStorage to sessionStorage in my case.

var _lsTotal = 0,
    _xLen, _x;
for (_x in sessionStorage) {
    if (!sessionStorage.hasOwnProperty(_x)) {
    _xLen = ((sessionStorage[_x].length + _x.length) * 2);
    _lsTotal += _xLen;
    console.log(_x.substr(0, 50) + " = " + (_xLen / 1024).toFixed(2) + " KB")
console.log("Total = " + (_lsTotal / 1024).toFixed(2) + " KB");

My firefox browser is currently holding 9.5 MB of session storage data.

I am wondering how firefox is able to handle it since I keep reading that the default size in 5 MB. How long it is going to be before firefox or any other browser is going to give up on this if the size of sessionStorage keeps on increasing.