NEW YORK - "Thor: Ragnarok" thundered to one of the year's best box-office debuts, opening with an estimated $121 million in North American theaters.

  

The robust debut for the third "Thor" movie was a welcome shot in the arm for Hollywood and theater owners who have just suffered through a terrible October at the box office.

  

"Thor: Ragnarok" also bucked the trend of diminishing returns for sequels. The 2011 "Thor" debuted with $65.7 million; 2013's "Thor: The Dark World" opened with $85.7 million.

  

The big opening cements the unlikely breakthrough of New Zealand director Taika Waititi, who shepherded the $180 million production to Marvel's best reviews since 2008's "Iron Man."

  

The weekend's other new nationwide release was the holiday-themed comedy sequel "A Bad Mom's Christmas." It grossed $17 million over the weekend.