Usually when you open the Library dialog to History (e.g., Ctrl+Shift+h), you see shortcuts for different periods of time. If you do not see those, then most likely your history was cleared. If you enter http in the search box at the upper right, do you get any entries? That should match almost everything. Users have reported occasionally that the dialog stops working correctly. I'm not sure whether this indicates a problem with the database file that stores your history and cookies, or whether it is some other problem in Firefox itself. Have you noticed any issues in that dialog with accessing your bookmarks?