Home icon menu layout affected when viewed in Mozilla

I am using images for my home icon menu layout. They are ordered in a grid – 3 per line, 9 in total.
They look completely fine when I view them in Chrome, but when I view them in Mozilla Firefox it ruins the layout of the last 3 icons. For some reason it pushes the first icon in the row all the way over to the right, which then pushes the other 2 image menus down to the next level.
I have been careful to use the same 400×266 image size for each icon. But I don’t think it’s an image size problem because even when I tried to use smaller images, it didn’t fix it.
I have also tried deactivating the few plugins I use, but this doesn’t help either.
https://relocatingtoireland.com/