For Britney Spears, nothing is stronger than yesterday.

“Britney is stuck in the ’90s,” a source close to the “Make Me” singer reveals in the new issue of Us Weekly. “She brings CDs and a boom box to photo shoots and fittings.”

The 34-year-old’s music selections include the Backstreet Boys, Nirvana, Madonna, En Vogue and Jewel, says another insider. “She loves all the stuff she grew up with,” adds the source. “She doesn’t listen to modern stuff. She’s never really grown out of the ‘90s.”

Though N’SYNC doesn’t appear on that list, Spears revealed during a Q&A on Most Requested Live with Romeo last month that she’d be willing to collaborate with ex-boyfriend Justin Timberlake. “Justin Timberlake is very good,” she said. Jamie Lynn’s big sis also named fellow ’90s stars Gwen Stefani and Steven Tyler as potential collaborators.

“He’s a genius, I think. He’s very rock ’n’ roll. He sings from his soul,” Spears declared of the Aerosmith frontman. “Gwen Stefani, I think she’s great.”

The “Can’t Stop This Feeling” crooner likes the idea. “Absolutely, absolutely,” Timberlake told E! News when asked at the Toronto International Film Festival September 13 if he’d be down for a duet.

The mom of Sean, 11, and Jayden, 10, (with ex-husband Kevin Federline) recently threw it back to her “… Baby One More Time” days by sporting a vamped-up version of her 1998 debut music video look at September 24’s iHeartRadio Music Festival in Las Vegas. Spears opened the night with a medley of her hits.

“Had so much fun at #iHeartFestival!!” she tweeted after taking the stage, adding the next day, “Performing with @G_Eazy was AMAZING”