I hope you did not uninstall an earlier version and use the option to remove personal settings, if you did then luck and timely use of an undelete utility will be required.

Otherwise see

• lost bookmarks
• you may also be interested in this thread: /questions/897484 Why, after upgrading to version 8, are all of my bookmark toolbar icons missing while they are still listed under "organize bookmarks/ bookmarks toolbar" ? (That thread is not solved but has some suggestions that may be applicable)

A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.com/kb/Bookmarks+not+saved#w_places-database-file