Firefox bug combining absolute and fixed positioning

[flicity]

Hey helpful people.

I am designing a site which has a floating toolbar (position: fixed) with popup menus. The popup menus are using nested lists and position: absolute to display.
JavaScript is dealing with the transition.

The problem is that in firefox alone the fixed toolbar jumps when the popup menus show and hide. Removing either position: fixed or the offset (bottom: 27px) removes this problem. However these need to be present in the design.

Anyone know a workaround? (webpage and css attached)