Does anybody have any suggestions on how I could make this work?
Yes, bug 380639 seems to have an attached code with a potential patch for this issue.
There's no easy way to fix this without mutation event listeners, which are expensive perf-wise. We could alternatively just rebuild the menu each time it's opened (which would also fix bug 380639), but that might be too slow.
Well, and whether it's a menubutton or a button depends on the (current) state of the world, and we have a healthy dose of bugs on the way we currently don't show that, so to not block off fixing them the feed list would need to be rebuilt for the tooltip's popupshowing and for click, not just the menu's popupshowing. Bleah, and it would still block half of showing, since the two things people want are tooltips that say "Subscribe to this page's feed" or "See a list of this page's feeds" and to have a dropmarker when there's more than one. But you might have multiple feeds onload, so we put a dropmarker on the icon, and then all but one is removed, so when you click the icon to see the menu, we grovel through the DOM and decide to remove the dropmarker and load a feed instead of showing the menu you expect.