I resolved my own problem. For future reference to anyone else, if you have a similar problem you may want to check display and positioning. In my case, my menu links were sitting inside another div container, and this parent container was set to display: inline. This caused the parent container to be squished to one side of my screen next to a search bar. So while the menu looked like it was in the right place, the parent div wasn’t (though only FF seemed to care about this). I removed the display: inline, and now it works like normal.
Have a nice day.
Happy days 🙂 I recommend that you mark this topic as resolved as well 🙂