VENTURA, Calif. — Fresh off a 24-hour shift in fire-filled valleys, Fabian Uresti, a veteran firefighter from California’s Central Valley, shrugged off the question of whether he would make it home for Christmas.

“I missed Thanksgiving this year, too,” Mr. Uresti said. “It’s part of the job.”

Major fires raging across the state this year from wine country to just shy of the Mexican border have stretched California firefighters to their limit.

“A lot of epic firefighting is occurring but we have a lot of tired folks out there on the line,” said Chief Janet Upton, a deputy director of Cal Fire.

Around 8,700 firefighters continued working on several fronts on Friday in Southern California as the largest of six major fires — 143,000 acres burned, with 10 percent containment — spread from Ventura County into Santa Barbara County. Officials said that fire, which has already claimed at least one life and destroyed 401 structures, was threatening thousands more buildings in several towns.