Bush cannot afford to fire Rumsfeld. Nor will Rumsfeld resign.
I'm not sure what the generals are trying to achieve. It could be to let the public know that a lot of senior military on active duty despise Rumsfeld and do not trust his judgment.
It could also just be disgust venting.
Rumsfeld at this point is already a lame duck SecDef. Congress doesn't respect his opinions enough to care about what he says on acquisition. The brass are digging in their heels to slow any changes he wants to make until after he leaves. He's already lost the fight to 'transform' the military - look at the QDR.
Rumsfeld was good for the military during his first 3 years or so - there was a lot of shaking up to do. However, he's outlived his usefulness. Unfortunately, he doesn't realize that.