We've been notified that this is a problem on Firefox's side with Flashplayer. It's not something that can be repaired on Forge of Empires' side. You can track the issue from here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1271398
If you're inclined on using a Firefox-based model, you could also try running the 32-bit version as that one does not have the problem, or maybe Waterfox in the interim till it is fixed.
While you cannot technically "downgrade" Firefox, you can look up lists of older versions, uninstall the current one (best to make sure you've saved any bookmarks and similar into a profile), and install an older version. I would not suggest doing this if you've not done it before and you have information (bookmarks and similar) that can might get lost. But you can get pretty good instructions
over here.