Yes it's rather disappointing that Blizzard nerfed Firelands while it is current content. But being current content is why they nerfed it. Let's look at some questions.
Have you killed N/H Rag?
If so then good for you, you killed him before the nerfs, you can already do the raid and its easy for you, nothing for you changes.
If not, why are you complaining. Now you have the chance to kill Ragnaros and see all the content of Firelands, so see the fights on heroic, and down them.
These nerfs were made so people who were struggling through Firelands could get through it, and see all that it has to offer in content while it's still current, and to help people when the next raid and 5 mans come out.
I'm sure the new 5 mans won't replace every slot with a new piece of gear, so having a 391 from heroic Firelands will help, especially since chances are the new 5 mans won't be higher in ILVL then 391.
Do you wanna face Deathwings powerful generals with an average ilvl of 360-370?
Cause I don't. I wouldn't want to spend the first 3 months of the Dragon Soul wiping on the first couple bosses cause I couldn't get through Firelands and get the better gear.
Standing in Org/Stormwind and seeing people in the good guilds wearing the good Dragon Soul gear is rather depressing when you know you can't get through the stuff.