This legendary quest item drops in both 10 and 25 of Firelands. The amount dropped varies across difficulty and mode, however.
It is as follows:
10 normal: 18-23 (7-8 weeks of full clears)
10 heroic: 23-26 (6-7 weeks of full clears)
25 normal: 48-55 (3 weeks of full clears)
25 heroic: 55-66 (3 weeks of full clears)
If you have a capable and reliable group, always farm 25 Heroic since it offers the highest dropped amount from the bosses. If you don't have a said group, then I recommend sticking to 10 Heroic at best.