Menu child items visible on Firefox when shouldn't be

Since the last update, since the theme options panel is gone and instead we are to use the WordPress Customizer, a lot of our customisations through the theme options are gone, and we’ve had to reproduce the previous colour scheme we had by editing our child theme and also finding that even if we tried to override the main.css with our changes, they wouldn’t ‘take’ without editing the main.css file of the parent theme.

We finally got through our customisations. We did not change any of the navigation menus other than colour changes. The menu works fine in Google Chrome and in Opera too, it exhibits the expected behaviour of the child items being hidden and not dropping down until the top level item is hovered over. In Firefox the child items are visible without hovering. Is there something we need to do to resolve this without breaking how it works in Chrome and Opera?
Site is at icarusstudios.co.uk