For the Bookmarks Toolbar, can you detect any pattern to the problem, or are bookmarks disappearing at random?

"I have tried to install various toolbar add-ons, all with the same result."

Were these add-ons directed toward fixing the bookmarks problem, or are these unrelated add-ons?

In some cases, Firefox's data file that stores toolbar settings, last window size, and other similar things, can become corrupted and needs to be reset or rebuilt. You can do that in Firefox's Diagnose Firefox issues using Troubleshoot Mode as described in this article: Changes to toolbars and window sizes are not saved.

I think it's always a good idea to back up before using Safe Mode, in case you accidentally wipe out your bookmarks or something. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

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.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

You can also try to repair the places database with this extension:

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