The couple, who are already parents to son Brooks, 4, and daughter Anja, 2, will welcome their third child this winter.
"We are so thrilled to add another member to our family," Firestone told the magazine. "I figure since our kids are already outsmarting us, they might as well outnumber us too!"
Before marrying Bozilovic in July 2008, Firestone proposed to Jen Schefft on the Season 3 finale of the ABC series in 2003. The couple split later that year.