I'm having a problem similar to what was reported in this thread earlier. I just upgraded to 8.8 and the form vanished from Firefox although it still appears in Internet Explorer and I can see it in Safari for iOS.
In Safari, if I subscribe an address I get "a confirmation message is on its way", but in IE when subscribing there's no feedback at all; the page just refreshes itself and the form is gone. It looks like there should be a message there.
I turned off the Ajax option like you recommended in the other thread and the form still hasn't returned. Do you have any suggestions?