First of all did you read the article about preparing firefox for testing -
Preparing Firefox for Web Testing http://support.smartbear.com/viewarticle/62858/and
Preparing Web Browser http://support.smartbear.com/viewarticle/62801/Second is your firefox patched -
Mozilla Firefox Patches http://support.smartbear.com/downloads/testcomplete/firefox-patches/Last the version of firefox supported by the TC version you are running. if not you may need to uninstall the current version of Firefox and install an older version supported by your TC version you are running.
There is a lot of information available here on the forum, in the online help, and in the software help articles.
Hope this helps you.