A good reason to not count on app-tabs as always being restored. Put your app-tabs into a bookmark folder so you can open them into tabs when you need to recreate them. See if you can get your app-tabs back via History > Restore Previous Session, then after that History > Restore previous Windows and then maybe restore previous tabs. I don't use sync and have no idea if it is involved in your problem. I do periodically close all of my tabs, close and restart Firefox, and then reconstruct my app-tabs by opening a folder into tabs. I had to number the titles in the folder so that the tabs would be in the order that I want them to be in as I do use the "Sort Places" extension to sort my bookmarks into alphabetical order.