The Boeing FCOM at my airline does indeed say that the first push of TOGA after takeoff will, among other things, remove the thrust "derates". The confusion lies in that at my airline we only use the Assumed Temperature method so that is all that will be removed.
It is always best to think of true derating as hanging a different size engine on the airplane, you can't very well go out and change that engine once airborne, at least not very easily. That differs from the AT method where you are just reducing the thrust setting. It is quite easy to push the thrust levers forward to recoup that reduction if necessary.
To add my two cents to the original question, no way would I add power for an engine fire indication. Engine fire does not equal loss of thrust. If anything you are struggling to get the nose down to level off at the MFRA with an engine fire, why exacerbate the problem with more thrust ?
Typhoonpilot