Gregory's Server@thunderbird I think most people whom are likely to use Thunderbird are probably Linux users. Your average Windows user probably just uses Outlook, if that (I suspect most people just check email in a browser).
Someone looking to use Thunderbird is likely invested in free software, and thus is likely to be on Linux. That is my logic.
@CyborgZeta But take a look at https://stats.thunderbird.net. Only a small fraction of our users are on Linux.