In the first of four posts, Aniston shared a pic of a smiling Cox, writing, "Happy birthday to this amazing human."
A pic of the ladies spending time together came next, as did one of Cox making a silly face. Aniston even apologized for being a day late in marking the occasion.
The last shot was a throwback from the women's time starring on Friends, on which Aniston played Rachel Green and Cox played Monica Geller.
On Cox's actual birthday, her other Friends co-star, Lisa Kudrow, shared photos of her and Cox snuggling with two dogs.
"Happy birthday @courteneycoxofficial," Kudrow captioned her post. "My genius generous gorgeous grounded great good FRIEND I love you"
Cox herself celebrated getting another year older by sporting a black bikini and diving into a pool with a giggle.
"Gracefully diving into this next year... #oaf," she captioned the clip.
Watch the video below for more on Cox's birthday celebration.