Why did Hashem refer to Yisrael as his firstborn son?
Rashi #1: "Bechori" means (not firstborn but) great (important). 1
Rashi #2: Hashem was substantiating that Eisav's sale of the birthright to Ya'akov was valid.
Seforno: Because they have a place of honor with Hashem, since they were the first to worship Him. 2
However this does seem to go well with the following Pasuk.
Seforno: Even though He expects all nations to call His Name (See Tzefanyah, 3:9). In a similar vein, the Meshech Chochmah explains that, just as a Bechor gives his father the status of a father, so too, were Yisrael the first to acknowledge Hashem and to give Him the status of Master of the world.