OP are you actually restoring TO an older firmware? I think you just restored the same firmware you had (which is older)... The title makes it seems like you downgraded to an older firmware, but I think you just restored the same old firmware... what is your original jb?
As mentioned earlier, this works on downgrading. It doesn't matter what firmware you have on your iphone. The first step iTunes takes is format the iphone.
That doesn't answer my questions regarding your restore....
Restored to the same firmware version?
What jb you had/used before doing the restore? 3GS you said?