Happy 30th birthday, Britney Spears! The pop icon and doting mother of two’s celebrity friends, from Justin Bieber to Christina Aguilera, helped to celebrate her big day on Friday by posting kind words on the “Hold It Against Me” singer’s Twitter page.“Happy birthday shoutout to my friend @britneyspears,” Justin Bieber posted on his Twitter yesterday afternoon.Fellow pop artist Rihanna expressed her love for the Mississippi native tweeting, “HAPPY BIRTHDAY @britneyspears love ya!”Christina Aguilera, who rose to fame around ... Read More >>