Exact same thing here. When I first login, if I just click on a panel menu first (like the power cog), items are highlighted fine. As soon as I click somewhere else though (like clicking a menu item from the cog menu, or clicking something in firefox) the bug manifests itself and I no longer get selections in gtk3 menus. It does work fine here in gtk2 apps as you described, no problems with firefox menus.
@Roman Yepishev (rye)
Exact same thing here. When I first login, if I just click on a panel menu first (like the power cog), items are highlighted fine. As soon as I click somewhere else though (like clicking a menu item from the cog menu, or clicking something in firefox) the bug manifests itself and I no longer get selections in gtk3 menus. It does work fine here in gtk2 apps as you described, no problems with firefox menus.