Strange fire is a metaphor.
Fire is a process. It is a chemical conversion, using oxygen to combine with the item being burnt, and the result is heat, hot gases (such as carbon dioxide) and ashes.
Strange = that which is foreign; that which is unrecognized.
In the kingdom of God, purity is recognized, and impurity (evil, corruption) is unrecognized. The unrecognized items must be rejected.
In the kingdom of God, processes happen by way of using our conscience to discern that which is right and good. Using the Law of God as a basis, the process results in a decision. The goodness of the Father in heaven requires that each of His servants must likewise decide according to that which is good. If a process of conversion of an offering from its grantor to its recipient is well-pleasing to God, then it is a good process, which should be called holy fire, but anything other than that which pleases God is strange fire.
If we read in the Bible on the topic of justice, it is clear that certain things are not right and good. For example, taking bribes destroys justice. Therefore, a bribe brought to the altar of Christ is an example of strange fire.