Jennifer Aniston turns 50: Ex Justin Theroux and more stars wish her a happy birthday

Happy birthday, Jennifer Aniston!

Everyone's favorite "Friend" turned 50 on Monday, and some of Hollywood's biggest stars -- and her closest friends -- took to social media to celebrate her milestone birthday (despite the fact that she doesn't have a public Instagram account).

SEE ALSO: Why Brad Pitt decided to attend Jennifer Aniston's birthday party

Perhaps most notably, ex Justin Theroux shared an Instagram post in honor of her big day, sharing an epic black-and-white photo of Aniston holding up bull horns over her head.

"Happy Birthday to this fierce Woman," he captioned the shot. "Fiercely loving. Fiercely kind. ....and fiercely funny. ❤️ you B."

Theroux wasn't the only ex of Aniston's that helped her ring in the big 5-0: Both Brad Pitt and John Mayer attended her star-studded birthday party at the Sunset Tower Hotel over the weekend.

Former "Friends" co-star and Aniston's best friend of two nearly thirty years, Courteney Cox, took to her new Instagram account to wish her a happy birthday, too.

"The world definitely got brighter the day this girl was born," she captioned a never-before-seen photo of them together. "Happy birthday sweetheart! I Love you!"

Reese Witherspoon, who stars alongside Aniston on the highly-anticipated upcoming Apple TV show, "The Morning Show," also took to social media to honor the big day.

SEE ALSO: Inside Jennifer Aniston's star-studded birthday party

"What a joy to work with this talented lady every day!" Witherspoon wrote. "She is gorgeous, hysterical and lights up the whole world with her sunshine. Happy Birthday, Jen! Also how does your hair look that good every day?? Seriously."

Take a look back at some of Jennifer Aniston's most iconic looks from throughout her career in the gallery below: