With Loeb switching to a part-time WRC programme for 2013 before turning his attentions to the World Touring Car Championship from '14, Hirvonen will become Citroen's main title hope.
Loeb goes into semi-retirement having delivered nine successive WRC crowns and led Citroen to eight manufacturers' titles between 2003 and '12.
Hirvonen finished runner-up to Loeb in this year's standings and won the Rally of Italy.
With Citroen yet to decide Hirvonen's full-season team-mate, Ford withdrawing its works backing, and Volkswagen's WRC Polo in its debut year, Hirvonen is an early title favourite for 2013, and is confident he can live up to that billing, particularly with the majority of the season again on his favoured gravel surface.
"I feel really, really good," he said. "On gravel I am sure I'm ready for the fight.
"It's going to be quite a different season, with Jari-Matti [Latvala] and Sebastien Ogier in a new team with Volkswagen - it's going to be exciting to see the pace of the new car.
"I'm really happy to be part of such a strong team. I'm going to be ready and I'm really looking forward to it."
The Finn's co-driver Jarmo Lehtinen said they had used the 2012 season as good preparation for next year's step up in status.
"The preparation has been mostly done for next year," said Lehtinen.
"We know the team and the car and we set ourselves targets. The first target was to know the car, the second was to deliver the manufacturers' title and the third was to put [team boss] Mr [Yves] Matton in a position where he had to give us team orders to slow down and we did this on a few rallies.
"I think we're ready for next year."
- Sports & Recreation