Ok, found the bug in the Firefox backlog: https://bugzilla.mozilla.org/show_bug.cgi?id=2060342
It’s a very general bug where all of the text with a rotateY or rotate3d style is broken ; so I can’t really fix it by changing the game since that’s used everywhere and kind of essential to the game UI.
The bug is going to be fixed in version 154 (the next release of Firefox), planned for August 18 (so in four days) - so I guess unfortunately you’ll just have to wait four days before using Firefox for playing it I guess ^^’