Unless this happens with menus that are drawn by Firefox, then it's definitely not a Firefox bug (but either a theme, Unity or gtk bug). Firefox has nothing to do with the appearance, positioning of layout of the window menus when they are drawn by Unity - it only controls the content (label + icon).
As Nautilus has the same issue, it's likely to either be a theme issue or gtk issue
Unless this happens with menus that are drawn by Firefox, then it's definitely not a Firefox bug (but either a theme, Unity or gtk bug). Firefox has nothing to do with the appearance, positioning of layout of the window menus when they are drawn by Unity - it only controls the content (label + icon).
As Nautilus has the same issue, it's likely to either be a theme issue or gtk issue