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We've gotten so used to Nicole Richie as a wife/mom/homebody, that it's nearly impossible to remember her hard partying days during The Simple Life era!

The former 31-year-old party princess left the kids and hubby at home as she paid a visit to one of her favorite old Hollywood haunts, The Viper Room, on Thursday night.

Richie has been working overtime on her hippie-chic fashion line, House of Harlow, and she's also lending her expertise as a "celebrity mentor" on the upcoming reality competition show, Fashion Star. With all that work, can see why this busy mamma of two needs to get out of the house, and blow off some steam!

Apparently Mrs. Madden hasn't been getting along with Fashion Star's other celebrity mentor, Jessica Simpson. The pair have reportedly been bickering non-stop since the show's taping began, with Nicole even making fun of Simpson's fuller figure.

Find out who is the biggest diva (Richie or Simpson) on the Fashion Star set, after the jump ...