Hi and thanks for the feedback :)
Indeed, when you add an item from another device and immediately open InMyPocket on Firefox, the new item is not immediately visible. I can't access the pocket service everytime the popup is opened in order to keep a sane amount of requests to the pocket service. However, in case you've not retrieved the list of items for 15 minutes or more, then the delta is always retrieved from pocket again, to make sure you don't keep too much of a difference between InMyPocket items list and your actual Pocket items. I agree it's a tradeoff and therefore not perfect, but I believe it covers most use cases :)

Do not hesitate to reach to me on @pabuisson or http://bit.ly/inmypocketcontact if need be, I'll be happy to answer.