User-agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.13) Gecko/20101207 Lightning/1.0b2 Thunderbird/3.1.7
False alarm... after 2 days of troubleshooting, it turns out we were
running into default "connection per minute" setting limits on the
remote ISA side.