Hello,
You probably have a conflict between Mozilla's ActiveX plugin and Flash (or any other ActiveX plugin).
Uninstall the Mozilla ActiveX plugin:
http://forums.mozillazine.org/viewtopic.php?t=206213#Uninstall
Here is some info on Mozilla's ActiveX plug in:
http://forums.mozillazine.org/viewtopic.php?t=206213#FF_1
Pay attention to the part that states: "Most methods of updating Firefox to a new version do not remove the ActiveX plugin, and there is no guarantee that the plugin version you have will work with the new version of Firefox. It is strongly recommended that you uninstall the ActiveX plugin before you update Firefox. Having the wrong version of the ActiveX plugin installed can cause other plugins (especially Flash) to malfunction."