Matthew 16:19; And I will give unto thee the keys of the Kingdom of heaven; and whatsoever thou shalt bind on earth shall be bound in heaven; and whatsoever thou shalt loose on earth shall be loosed in heaven.
Tithing is one of the keys that Jesus alludes to in Matthew 16:19. A key is any command or directive from God that has a promise or blessing attached to it. For example...Matthew 6:33 says, "Seek firs the kingdom of God and His righteousness, and all these things shall be added unto you." So; seek the kingdom (the directive) and you'll receive from the Lord all the things you need (the back end blessing.)
In Proverbs 3:9 the Bible declares, Honor the Lord with they substance and with the firstfruits of all thine increase: So shall thy barns be filled with plenty, and thy presses shall burst out with new wine.
Firstfruits and tithes are synonymous. Tithe means a tenth; out from, a part of.
Tithes are known as firstfruits because they are the tenth part (of all your increase) that comes out of your increase. God promises us that if we honor him with our substance and firstfruits, that he would cause our storehouses (bank accounts, investments, and all that pertains to us) to be filled with plenty.
God even goes as far as to say in Malachi 3:10 that we can put Him to proof with our tithes-to see if he will or will not open heaven and bless us for honoring Him with our tithes.