Was at a  site that opens pop-under windows(ADS).
These windows open sans Bookmarks Toolbar.
Closed main Firefox window.
Then closed a pop-under window(AD).
This AD was what opened the next time i used Firefox.
Opened a new page but the Toolbar did not return.

 The Bookmarks Toolbar would display for a second
and then disappear.
 The menu-item---
View/Toolbars/Bookmarks Toolbar was checked.
 Reinstalling Firefox seemed a bit extreme.

 A user somewhere (can't find it now) suggested this,
 but had not tried it.   I tried it. It worked.
Thank you obscure user!.
Opened Options in the Menu
(rightmost icon on the user bar)
under General -- Startup
 unchecked   __Restore previous session 
Closed and reopened Firefox; et voila,les Bookmarks! 
All +/-1700 of them.
Opened a page and then 
rechecked  __Restore previous session

Because this question appears in numerous guises have reopened it here with a solution...... What had happened: Was at a site that opens pop-under windows(ADS). These windows open sans Bookmarks Toolbar. Closed main Firefox window. Then closed a pop-under window(AD). This AD was what opened the next time i used Firefox. Opened a new page but the Toolbar did not return. What was happening: The Bookmarks Toolbar would display for a second and then disappear. The menu-item--- View/Toolbars/Bookmarks Toolbar was checked. Reinstalling Firefox seemed a bit extreme. What did happen: A user somewhere (can't find it now) suggested this, but had not tried it. I tried it. It worked. Thank you obscure user!. Opened Options in the Menu (rightmost icon on the user bar) under General -- Startup unchecked __Restore previous session Closed and reopened Firefox; et voila,les Bookmarks! All +/-1700 of them. Opened a page and then rechecked __Restore previous session The universe is as it was.......