Of course, I'm touched that you dropped by at all to say happy birthday. PPPL bros for lyfe. Happy belated birthday to you too! (You're closer to your last birthday than your next one, so I think I can still say it... )
Miyu Edelfelt from the manga Fate/kaleid liner PRISMA☆ILLYA, a magical girl spinoff of Fate/stay night starring Illyasviel. While it does have its serious parts, it's mostly a parody of the genre... though I really should point out it definitely earns its ecchi label.
Amusingly enough, I had only found out about it because of the crossover with Nanoha from a few years back. Anyway, it's been a couple years since I read it last, and the main reason I switched to a Miyu avatar is because I recently found out about the anime adaptation... okay, so the main reason is because I actually like Miyu, but that was what made me want to actually try and find a decent enough pic to use as an avatar.
I hope that answers your question, and I'm sorry if this was a bit longer than you wanted from an answer.
*e* Hm... may I ask the same question of your avatar? That looks familiar, but I can't quite place why.