Do you have the McAfee Site Advisor? That is known to cause this problem, if you uninstall or disable the McAfee Site Advisor the add-ons bar should remain hidden.

If you do not have the McAfee Site Advisor, check to see if another add-on is causing this, for details of how to do that see https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

When I went to uninstall/disable the Site Advisor, it said I would be uninstalling my subscription for getting updates. I don't want to do that, I just want to hide that horrid Add-On bar permanently (McAfee does regular scans that give me all the notification of updates that I need)!

OOPS, sorry, first-time user apology. I meant this to be more on the problem, not a possible solution, can't figure out how to correct my mistake.

Place your cursor on the bar. Right click. Go to customize. In the left hand bottom corner click text, then done. It will remove the icon and text. You will still have the bar but nothing will be displayed. It does not disable McAfee.