Steve Cotterill's side moved five points clear of safety with victory over the Lions and will now look to take another huge step towards securing their Championship status.
"It was a difficult game for us because Millwall have had some great results of late and they are a very dangerous team," said the City Ground boss on Saturday.
"They are honest, work hard and are not short of ability. To score three goals against them could not have been better."
Derby's inconsistent form meanwhile continued with a home defeat at the hands of Watford that left Nigel Clough to rue a slow start from his side.
"The second half we played well but we shouldn't be trying to come from behind all the time and although we played well, it's difficult when you are chasing the game to that degree."
Check out the head-to-head records of the two sides by clicking below.