Microsoft and Mozilla send each other cakes or cupcakes whenever they release new versions of their browsers. Mozilla recently celebrated its 15th anniversary, so Microsoft decided to send its competitor a picture of a new birthday cake, just to make sure the tradition goes on. Microsoft also thanks Mozilla for their continued advocacy for web standards.
"@IE: Congrats @Mozilla on your 15 year anniversary & your continued advocacy for web standards! @firefox This one kinda snuck up on us. Had to improvise," Microsoft's Internet Explorer team stated on Twitter.