Why does the Torah insert the Mitzvah of Bechor here?
Seforno: After presenting the Mitzvos of Chesed Bein Adam la'Chavero, 1 the Torah discusses Mitzvos that concern thanking Hashem for His kindness, 2 beginning with Bechor - which is befitting for the owner of the flocks to give the One who gave them to him. 3
Ba'al ha'Turim: To compare Eved Ivri to Bechor - regarding 1. the prohibition of making him work (the work of a slave), just as one may not work with a Bechor and 2. the prohibition against piercing the ear of an Eved Ivri who is a Kohen because it renders him a Ba'al-Mum, just as a Bechor that has a blemish is Pasul.
Seforno: Regarding produce - Ma'aser Ani; money - Shemitas Kesafim and Tzedakah; and other acquisitions - the Ha'anakah of an Eved Ivri.
Seforno: Including: Thanking Hashem for taking us out of Egypt - via the Yom-Tov of Pesach incorporating the Korban Pesach and Matzos; The ripening of the produce - via the Korban Omer; The harvest - via the Yom-Tov of Shevu'os; and The ingathering of the produce via the Yom of Succos - and by giving Hashem a gift on each occasion, as the Torah writes at the end of the Parshah "ve'Lo Yera'eh es P'nei Hashem Reikam".
How will we reconcile this Pasuk ("Kol ha'Bechor ... Takdish") with the Pasuk in Vayikra 27:26) ""Ach Bechor asher Yevukar ... Lo Yakdish ... "?
Rashi #1: You may not declare it Hekdesh as another Korban, but you can and should, declare it Hekdesh as a Bechor.
Rashi #2: You may not declare it intrinsically Hekdesh, but you may declare it Hekdesh Iluy - to give its value to Hekdesh. 1
Rashbam: "Takdish" means treat it with Kedushah, whereas "Lo Yakdish" means that one may not declare it Hekdesh as another Korban.
Rashi: In which case, he pays Hekdesh according to what the animal is worth to him (according to the stakes he has in it). See Sifsei Chachamim.
What will be the Din regarding the reverse case - working with a firstborn lamb and shearing (the tail of) a firstborn calf?
Rashi (citing Chulin, 137a): That too is equally forbidden, and the Torah presents the more common case.