If I restart Firefox with Add-ons disabled, the toolbar doesn't load - it is also disabled then.
It would have been simpler just to disable the Avast AOS add on (extension).
I personally think you should update (via a complete uninstall/cleanup/reinstall) to 2014.9.0.2011, however that is up to you.
I am running 2014.9.0.2011 on Windows 7 Pro, 64 bit, with Firefox 26 and I don't have the issue you describe. I am reasonably sure your problem is with your Firefox installation, and not Avast.
My first suggestion would be to simply try resetting the Firefox profile - see
https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problemsIf you want to remove the Avast brower AOS add on from the issue altogether, you can:-
Goto Start->Control Panel->Programs and Features.
Double click on Avast! Free Antivius
If you get a User Account Control dialogue, click on yes.
Click on Change, then Continue
Untick Browser Protection, then click Continue
edit: You can also untick Browser Cleanup if you likeI personally don't have AOS installed - in my view it causes more issues than it's worth.