Exodus 30:34-38 explicitly says that the mixture of ointments to create the incense to be offered was “holy to the Lord” and was not to be used for personal purposes – only for worship.
In addition, Ex 30:9 explicitly says “Ye shall offer no strange incense thereon, nor burnt sacrifice, nor meat offering; neither shall ye pour drink offering thereon.” (KJV)
Numbers 16:46 indicates that this incense was to put in the censer along with fire from the altar and then taken into the altar of incense.
Now, was their fire a “strange fire” because they got the fire somewhere other than from the altar, or was it because they decided not to bother with the proper mixture of the incense, we’re not told. The important thing is that this presentation was a package deal (fire and incense) at the altar of incense, and was called a “strange fire”. And because of their arrogant disregard for God’s will, they were condemned for their disobedience.