okay so it was totally my bad for not testing this properly on other browsers HAHA sorry about that and thank you for bringing it to my attention T___T i'm prettyyy sure this is an issue with how firefox renders the material icons google font though i'm not 100% sure... 😔

anyway the easiest solution i can find atm is to look up: "text-indent: -999px;" in your code and replace it with "text-indent: -24px;" — from what i tested, this should let it work on both chrome + firefox but please let me know if that's still buggy or if you're having issues!