Here the word "gifts" refers to the sacrifices that the people made to the idols that they worshiped. AT: "because of their sacrifices" or "because they gave gifts to idols" (See: [[rc://en/ta/man/translate/figs-explicit]])
The phrase "firstborn of the womb" refers to the firstborn child of a woman. The phrase "made a sacrifice" can be translated with the verb "sacrifice." AT: "they sacrificed every woman's firstborn child" (See: [[rc://en/ta/man/translate/figs-explicit]])
It is implicit that they caused their children to pass through the fire in order to offer them as burnt offerings to their idols. AT: "sacrificed them as burnt offerings" (See: [[rc://en/ta/man/translate/figs-explicit]])