So far, this seems specific to Facebook private messages, but I can not confirm that. Perhaps it affects any Gigaboxx fields?
1) Login to Facebook
2) Navigate to any friend's profile
3) Click "Send <friend> a message" on the left menu
4) Attempt to fill out message fields; Subject is OK, Message will not accept text
*** Bug 29925 has been marked as a duplicate of this bug. ***
I'm going to "git bisect" to see if I can find what caused this.
This broke in r48698.
That change was related to bug 29292.
*** This bug has been marked as a duplicate of bug 29815 ***