It looks like Apple updated the code to detect which client you are when connecting to the app store.
I checked and they have script that called a function its.detect.Itunes which detects what browser and operating system you are on, and if it isnt Safari, the link does not return an app store link.
Interestingly it does seem to have handlers for chrome and firefox, but either they made a mistake or they are purposefully making it so you cannot use Firefox to one click browse the mac app store.
I checked if switching firefox's user agent worked, and while it did load the page momentarily, and the function clearly had a anchor link instead of a direct href, it immediately redirects to a blankly loading page forever.
At this point it looks like it is probably safari or bust.