Heh, an interesting start.  Looking forward to see where this one is going.

The cider obsession thing from Super Cider Squeezy creeped me the heck out.  

Huh, for a non-native English speaker you've managed to go the entire chapter without any noticeable mistakes, great job!  

Also, "Suddenly Mjölna´s eyes shot open, their blissful gleam fading as she looked at Pinkie Pie. The pink mare's face was frozen in an awkward, glassy-eyed grin as Pinkie tried to wrap her mind around Mjölna's love of the straw's hard, eager length."

Hey there! Let's give you a review of the first chapter, shall we?

I really like how you are going to use Mjörnla's character, as she isn't popular enough with the fandom to begin with. She's a good pony, I like her color scheme, but I don't completely like how you take on her personality. So far she only has one character trait: She likes cider. Besides that, not much. I know you will develope her more in future chapters, and I am eager to see those.

Also, it might just be me, but I feel so awkward when reading what goes through her mind when she's drinking the cider. It's almost as if she's sexually enjoying it! It must just be me; I have a pervy mind.