Unfortunately, it often happens that users do not update their browsers. Maybe its possible to change the code and make the menu compatible with earlier versions. Thank you very much.
Ah, that is a very old version (4 years old). Firefox auto-updates in most cases, so it’s likely that a very small percentage of people are still using that version.
Your header is using Flexbox, which was supported at that time. What happens if you remove a menu item? Does it fix the issue?