Let's try some science. We need a control, so lets create a blank
Firefox profile. This requires running firefox with a command of
'firefox -P'. This will bring up the profile window and then you can
create a blank profile and try to set your proxy to use Tor and try it
again, and then try non-Tor. Then we can see if you get the same exact
results, or if your old profile got damaged by one of your addons (it
can happen).
If the issue does *not* happen with a fresh profile, try adding your
addons back one at a time until it does. Then maybe we can get
somewhere.
http://support.mozilla.com/en-US/kb/Managing%20profiles
http://kb.mozillazine.org/Profile_Manager