It's not "Firefox-only" per se, it's CSS. Firefox is fast when it comes to implementing updates that benefits multilingual and Asian support, and Chromium is either slow, implements a small part only, or just ignores it completely.
(aside: Another good example is Ruby annotation. Firefox's implementation of Ruby is up-to-date while Chromium's stuck in 2010.
And this is very very annoying, you have to design for Chromium when it comes to Ruby annotations; or use JavaScript to serve different Ruby codes per browser. Chromium is practically the "modern IE6".)