After setting layers.offmainthreadcomposition.async-animations to true, I am no longer able to interact with the right click menu in about:config with the mouse, I'm stuck having to use the arrow keys to move around the menu and then having to use the enter key to make my selection. Other menus in the Browser chrome don't seem to be affected, however, text dropdown menus here on Bugzilla are affect and I can't select anything in those menus with the mouse. Seems to be hit or miss around the Web as to what kind of drop down menus are affected by this issue. I see nothing in the Browser Console either.