There is a css display bug for me in firefox 49 that makes the editor title unreadable.

I had a fiddle with firefox developer tools and narrowed it down to the padding attribute of the
.ReplyEditor__title
class in this css file .
If I toggle that off it displays fine :

I saw the same issue in the test site when I first tested, but didn't report it (sorry!) - and it only seems to happen in firefox as @krystles machine with chrome is unaffected on both the normal and test sites.
Shit happens , hopefully not many people are affected by this - in the meantime you can fix it with developer tools or just copy/paste your title in from elsewhere :)
Edit: Confirmed by a few users, opened a github issue here