There appears to be a compatibility issue between the latest version of Firefox 55.0.3 (32-bit), Flash Player 26.0.0.151 (32-bit) and the family tree view on Geni.com (running on Windows 10).
Family trees are displayed but the application interface does not respond to any mouse inputs (movements or left mouse button presses). A right mouse button click shows the Flash menu that allows checking of the Flash settings, which then become unresponsive, ignoring mouse input and not closable (Firefof 55.0.2 did not have these issues). The Flash settings seem to be ok, and other websites with Flash content work without issues.