doubt from friction

Consider a case when a block is placed on a horizontal platform at rest. It then starts accelerating with a constant acceleration such that,
Friction present between the block and the horizontal platform > mass times its acceleration
so clearly sliding won't take place..
but can someone tell me how with reference to the fixed frame is the work done by force of friction +ve???

does it has to take something with "limiting friction"....

and what is this "fixed reference frame"....

please help...!!