Update -- once I manually remove the add-on bar, it now stays off. It doesn't appear in new windows, and when I restart Firefox it remembers from the previous session. So, I guess that's success of a sort.

IF YOU HAVE FIREBUG INSTALLED!!!!!!!!! Its easy, just open a new tab and type: 'about:config' hit enter and you will be shown a warning if you have not yet disabled it and just click, 'i'll be careful, i promise!' button. Now, in the configuration tab, click to the filter and type: 'addonbar' you'll get a result like this, somewhere: 'extensions.firebug.addonBarOpened' CHANGE THIS TO FALSE. And, you're done. (restart FF)