Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac Options) key, and then starting Firefox. Is the problem still there?

Start your Computer in safe mode with networking. Then start Firefox. Try Safe web sites. Is the problem still there?

Starting The Computer In Safe Mode;
Free Online Encyclopedia

You can check for problems with the places.sqlite database file (bookmarks, history) in the Firefox profile folder.

• Places Maintenance: https://addons.mozilla.org/firefox/addon/places-maintenance/

If the Places Maintenance extension can't repair the places.sqlite database file then remove all places.sqlite file(s) in the Firefox profile folder to make Firefox rebuild the places.sqlite database file from the most recent JSON backup in the bookmarkbackups folder.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file