The firmament of heaven represents mutual love. The firmament, as in Genesis 1, signifies the internal man.

(References: Arcana Coelestia 24)

51. EACH SOCIETY IS A HEAVEN IN A SMALLER FORM, AND EACH ANGEL IN THE SMALLEST FORM.

Each society is a heaven in a smaller form, and each angel in the smallest form, because it is the good of love and of faith that makes heaven, and this good is in each society of heaven and in each angel of a society. It does not matter that this good everywhere differs and varies, it is still the good of heaven; and there is no difference except that heaven has one quality here and another there. So when any one is raised up into any society of heaven he is said to come into heaven; and those who are there are said to be in heaven, and each one in his own. This is known to all in the other life; consequently those standing outside of or beneath heaven, when they see at a distance companies of angels, say that heaven is in this or that place. It is comparatively like civil and military officers and attendants in a royal palace or castle, who, although dwelling apart in their own quarters or chambers above and below, are yet in the same palace or castle, each in his own position in the royal service. This makes evident the meaning of the Lord's words, that

In His Father's house are many abiding places (John 14:2);

also what is meant by

The dwelling-places of heaven, and the heavens of heavens, in the prophets.