That one used to trigger on Thunderbird and Apple Mail. It was fixed after I submitted the relevant bug reports.
Gmail does much much better - a counter-intuitive result.
For those interested: https://grepular.com/DNS_Prefetch_Exposure_on_Thunderbird_an...
I am surprised that you're seeing leaks in Thunderbird 10.0.2. That is my own client of choice so I'm always keeping an eye on it after updates and I am not currently seeing any leaks... If you have remote images disabled, are you still seeing leaks? If so, which ones?
I disabled preFetch in the config editor and my ship's running dry again.