In Firefox the M1 Porto demo site has some JS Warnings which may be the culprit. A return is in the following method as well as subsequent code which could explain the difficult in finding the elements absolute position to scroll to on validation errors.

/*
 * Different ways to find element's absolute position
 */
Calendar.getAbsolutePos = function(element) {

    var res = new Object();
    res.x = 0; res.y = 0;

    // variant 1 (working best, copy-paste from prototype library)
    do {
        res.x += element.offsetLeft || 0;
        res.y += element.offsetTop  || 0;
        element = element.offsetParent;
        if (element) {
            if (element.tagName.toUpperCase() == 'BODY') break;
            var p = Calendar.getStyle(element, 'position');
            if ((p !== 'static') && (p !== 'relative')) break;
        }
    } while (element);

    return res; // <-- B00MER NOTE: this shouldn't be returning res here, return only AT end of method or some code can become unreachable by the browsers DOM implemenation.

    // variant 2 (good solution, but lost in IE8)
    if (element !== null) {
        res.x = element.offsetLeft;
        res.y = element.offsetTop;

        var offsetParent = element.offsetParent;
        var parentNode = element.parentNode;

        while (offsetParent !== null) {
            res.x += offsetParent.offsetLeft;
            res.y += offsetParent.offsetTop;

            if (offsetParent != document.body && offsetParent != document.documentElement) {
                res.x -= offsetParent.scrollLeft;
                res.y -= offsetParent.scrollTop;
            }
            //next lines are necessary to support FireFox problem with offsetParent
            if (Calendar.is_gecko) {
                while (offsetParent != parentNode && parentNode !== null) {
                    res.x -= parentNode.scrollLeft;
                    res.y -= parentNode.scrollTop;
                    parentNode = parentNode.parentNode;
                }
            }
            parentNode = offsetParent.parentNode;
            offsetParent = offsetParent.offsetParent;
        }
    }
    return res;

    // variant 2 (not working)

//    var SL = 0, ST = 0;
//    var is_div = /^div$/i.test(el.tagName);
//    if (is_div && el.scrollLeft)
//        SL = el.scrollLeft;
//    if (is_div && el.scrollTop)
//        ST = el.scrollTop;
//    var r = { x: el.offsetLeft - SL, y: el.offsetTop - ST };
//    if (el.offsetParent) {
//        var tmp = this.getAbsolutePos(el.offsetParent);
//        r.x += tmp.x;
//        r.y += tmp.y;
//    }
//    return r;
};

SEE INLINE COMMENT NOTE IN ABOVE CODE.

More details as to what could possibly be band-aids for other lesser browser version support that is breaking others.

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Stmt_after_return

What went wrong?

Unreachable code after a return statement might occur in these situations:

When using an expression after a return statement, or
when using a semicolon-less return statement but including an expression directly after.

When an expression exists after a valid return statement, a warning is given to indicate that the code after the return statement is unreachable, meaning it can never be run.

Why should I have semicolons after return statements? In the case of semicolon-less return statements, it can be unclear whether the developer intended to return the statement on the following line, or to stop execution and return. The warning indicates that there is ambiguity in the way the return statement is written.

Warnings will not be shown for semicolon-less returns if these statements follow it:

throw
break
var
function

Other code unreachable as well:

reloadPrice: function(){
    var price    = 0;
    var oldPrice = 0;
    for(var i=this.settings.length-1;i>=0;i--){
        var selected = this.settings[i].options[this.settings[i].selectedIndex];
        if(selected.config){
            price    += parseFloat(selected.config.price);
            oldPrice += parseFloat(selected.config.oldPrice);
        }
    }

    optionsPrice.changePrice('config', {'price': price, 'oldPrice': oldPrice});
    optionsPrice.reload();

    return price;

    if($('product-price-'+this.config.productId)){
        $('product-price-'+this.config.productId).innerHTML = price;
    }
    this.reloadOldPrice();
},

Also, it may help if known Browsers with issues & if any without? Ultimately best approach is to get support from the Porto authors to see if there is a fix and/or if others experience similar issues.

Hope this helps!