Firefox uses this Google “bad list” as described in my prior posts: http://en.wikipedia.org/wiki/Google_Safe_Browsing
The Mods have probably reported the thread in question as being “fixed” now, using this Google form: https://www.google.com/safebrowsing/report_error/?tpl=mozilla
It was Google’s search engine that returned a link to a warning page that the requested URL was unsafe.
Google flagged the particular FR thread as unsafe because of the remote image link someone inserted in a reply.
So if you search Google (including the site name freerepublic in the search) for the FR thread in question the first link to the thread it returns is marked as unsafe from the get go meaning your browser isn’t doing the redirection to the warning page. It was embedded in the search result link Google provided directly.