This has been happening for at least several weeks. Whenever I get an app update notification and dismiss it, whether by swiping it away or by clearing all notifications, Fennec comes to the foreground, which is clearly not the desired behavior. This is on Android 4.4.4 with a recent Aurora.
Kind of reminds me of bug 1075476.
Per bug 1235869, we're going to disable the Android web runtime, so we won't fix this bug in it. (This is part of a bulk resolution of bugs in the Firefox for Android::Web Apps component, from which I attempted to exclude bugs that are not specific to the runtime, but it's possible that I included one accidentally. If so, I'm sorry, and please reopen the bug!)