You have installed these 3 same extensions:

• Version 0.9.16, copyright 2008-2010 Veetle Inchttp://www.veetle.com/
• Version 0.9.18, Copyright 2006-2009 Veetle Inchttp://www.veetle.com/
• Version 0.9.18, copyright 2006-2010 Veetle Inchttp://www.veetle.com/

IF these extensions are not useful, Remove all of them OR Remove the older ones.

Uninstalling an Add-on

• https://support.mozilla.com/en-US/kb/Cannot%20uninstall%20an%20add-on

Also see this - Troubleshooting extensions and themes

• https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes

Check and tell if its working.

Thanks for replying, mha007. Sorry for the confusion of the three extensios which look the same because they aren't shown in full name here.

I found out one solution in Mozillazine that this problem is caused by the corrupt file "places.sqlite" in profile folder. See here: http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox

But the solution is only for a temporary help. Because after I finished installing a new add-on (I've tried some different add-ons to see if it's the problem of certain add-on, and it's not), the toolbar I created would still be gone every time I install a new add-on......

Also, I tried to remove Firefox entirely, but after reinstalling the problem still exists.

I think I found the main problem... but it's strange why it didn't work at first I stopped all add-ons and reinstalling Firefox.

I remove the add-on "Browser Backgrounds 2.2" and the missing toolbar just come back...

Glad to hear it

I can't click "Solved It" on my own reply... sorry.

The link to Troubleshoot your Add-ons is mentioned in my first reply.