Jamieson, Fausset and Brown on Deuteronomy 33:2-4

2-4. The Lord came--Under a beautiful metaphor, borrowed from the dawn and progressive splendor of the sun, the Majesty of God is sublimely described as a divine light which appeared in Sinai and scattered its beams on all the adjoining region in directing Israel's march to Canaan. In these descriptions of a theophania, God is represented as coming from the south, and the allusion is in general to the thunderings and lightnings of Sinai; but other mountains in the same direction are mentioned with it. The location of Seir was on the east of the Ghor; mount Paran was either the chain on the west of the Ghor, or rather the mountains on the southern border of the desert towards the peninsula [Robinson]. (Compare Jud 5:4, 5; Psa 68:7, 8; Ha 3:3).

ten thousands of saints--rendered by some, "with the ten thousand of Kadesh," or perhaps better still, "from Meribah" [Ewald].

a fiery law--so called both because of the thunder and lightning which accompanied its promulgation (Ex 19:16-18; De 4:11), and the fierce, unrelenting curse denounced against the violation of its precepts (2Co 3:7-9). Notwithstanding those awe-inspiring symbols of Majesty that were displayed on Sinai, the law was really given in kindness and love (De 33:3), as a means of promoting both the temporal and eternal welfare of the people. And it was "the inheritance of the congregation of Jacob," not only from the hereditary obligation under which that people were laid to observe it, but from its being the grand distinction, the peculiar privilege of the nation.