"When Firefox 2.12 came out on Feb. 7, it brought with it fixes for three critical security holes and seven that were not quite so serious. According to the security advisories, many of these problems were also fixed in the Thunderbird 2.12 e-mail client. Unfortunately, there is no Thunderbird 2.12.
"The Mozilla Foundation's press release focused on the Firefox 2.12 security fixes. The Foundation also reported, though, in its MFSA (Mozilla Foundation Security Advisory), that these same bugs had been fixed in the fictitious Thunderbird 2.12..."
