What does "FB" mean? I can only guess it's some kind of protective device because of F, like with FU for fuse.
Is there any comprehensive online guide for such kind of symbols?
From your description, sounds like an abbreviation for Ferrite Bead.
FB is the designator for a ferrite bead. A FB acts a lot like an inductor, in that it filters out AC, and they slow transition of DC signals.
Best list of designators I've run across is in Wikipedia.