This has happened to me before in FireFox, for some reason when I tried to sign on one day it asked me to open in default user profile or to create one. When I tried to open the default user profile it said it was already running, I had to create a new profile thereby deleting all of my bookmarks.
But yesterday when I clicked on to firefox it asked me to pick a profile and I chose the first default one I had and got all my bookmarks back. But I couldn't figure out myself how to switch profiles.