Thank you for the question on tithing, “Is tithing a Pentecostal Church doctrine or just one of the modern mega churches?”
The doctrine of tithing is prescribed by God to the Jewish people. They were to give a tenth of their harvest or work to the LORD God in supporting the Temple worship. With the destruction of the Temple, the giving of tithes were given to the synagogue and for Christians the practice became giving to the church to support the workers of the church.
As far as I understand the New Testament, the giving of the tithe is not mandated as one is give willingly and cheerfully, not out of compulsion. See Acts 2:42–47, II Corinthians 8–9.
There are local churches and denominations that mandate 10 percent tithing of its members. This is a matter of church policy more than a biblical junction.
SUMMARY: Tithing is a command by God to the Jewish people, while “first fruit” is the free will offering by Christians to the local church in their expression of their love for God.