It's hidden away now, you have to open the "Page Info" window, available most-readily from the page's context menu, then go to the Feeds tab, select a feed and hit the subscribe button that appears, and it'll take you to the feed itself, where Fx shows its subscription interface (which you can get to directly if the page is helpful enough to provide its own link to the feed, as I think WordPress used to do by default in its meta links section back in the day).

However there's a distinct possibility said subscription interface will go away in future. (Whether or not the list of feeds in Page Info would go away at the same time is less clear)