Why is the script set to function on ALL domains, rather than targeting store.steampowered.com and steamcommunity.com?

Not working for me. Firefox 50.0.2.

can you do it somehow without it being a popup?

doesn't work.

Does this support regional links (not available in region titles, Steam bundles removed from certain regions during sales)?

Can you add function so it also redirects when the game's region locked?

I went ahead and played a little around with your code, trying to eliminate the need to wait for an ajax response every time.
What my code does different, is that it always opens the target url in a new tab first.
Then it fetches the page again in the xhr and does the redirect check. If it was redirecet, the previously opened window is closed and a new tab is opened with the steamdb link.

This way, if it is a valid url, everything just works. If not, you'll see a tab open and close in <1s before you get sent to the right page.

document.querySelectorAll("a[href*='://store.steampowered.com/app/'], a[href*='://store.steampowered.com/sub/']").forEach(function(linkElem) {
    linkElem.onclick = function() {
        var w = GM_openInTab(linkElem.href);
        GM_xmlhttpRequest({
            method: "HEAD",
            url: linkElem.href,
            onload: function(response) {
                if (/^https?:\/\/store\.steampowered.com\/?$/.test(response.finalUrl)) {
                    w.close();
                    GM_openInTab(linkElem.href.replace("store.steampowered.com", "steamdb.info"));
                }
            }
        });
        return false;
    };
});

Firefox, script unfortunately not working.

It works on Firefox 52. But not older version like Firefox 39.

I modify some code that compatible with Firefox 39 without Jquery.

var eles = document.querySelectorAll("a[href*='://store.steampowered.com/app/'], a[href*='://store.steampowered.com/sub/']");
for (var i = 0; i < eles.length; i++)
{
    eles[i].addEventListener("click", function(ev)
    {
        ev.preventDefault();

        var linkElem = ev.target;
        var w = linkElem.target ? unsafeWindow.open(linkElem.href, linkElem.target) : unsafeWindow.open(linkElem.href, "_self");
        GM_xmlhttpRequest({
            method: "HEAD",
            url: linkElem.href,
            onload: function(response) {
                if (/^https?:\/\/store\.steampowered.com\/?$/.test(response.finalUrl)) {
                    w.location.href = linkElem.href.replace("store.steampowered.com", "steamdb.info");
                }
            }
        });
    });
}

PS: document.querySelectorAll("...").forEach can only use in FF50+ https://developer.mozilla.org/en/docs/Web/API/NodeList#Browser_compatibility

Thanks for supporting middle click.

I have a suggestion.
Please restore attribute href after click.
It's important to me to identify each links are visited.

And I was wondering, why left mouse button stopped to work correctly... No thanks, some redirects don't worth broken browser behavior.

Possible to don't removeAttribute("href") when right click?

-

I just found it when it's broken :(

Doesn't work any more.