If it's deleted, I don't think you can do anything about it.
I don't know if Firefox stores it in a temporary place or anything after deletion.
Do you have any other siblings?
They might have been looking at something they shouldn't have. :3
But, unfortunately, you'll just have to find things manually.
Does it continue to delete things, say, if you quite firefox and open it up again?
If so, you might have toggled the auto-delete on somehow, or not even done it yourself. Mines deleted mine everytime I close firefox since I first downloaded it onto this PC. Never selected it. Could undo it, but I can't be bothered.