This issue appear due to a website's behavior. For example, if you access a non-social website which has an embedded Social Network connection (i.e: Twitter or Facebook), the whole website will be blocked. The HTTPS connection that the main website creates for the embedded Social Network will be detected by Trend Micro NSC Firefox Extension.
The reason why IE does not block this kind of websites is because of the IE NSC extension limitation. IE extension can only block the exact URL typed in the address bar, so the https connection embedded in the web page cannot be scanned.