Related question: Years ago, when I was still a Windoze user, I learned that by not using Outlook Express AT ALL, sharply reduced my risk of nasties - regardless of what neato paint job I had on it. Eudora for example, piggybacked Outlook Express. But it was just cargo riding the same potentially-infected train.
Unless I've been drinking ENTIRELY TOO MUCH, I see Thunderbird as being web-based rather than resident, and therefore less susceptible to infection by nasties.
Granted, "nasties" + "Linux" = "oil" + "water" (generally speaking), but still....