Top of my wishlist : a contact autocompletion in #Thunderbird 's To: field, that would suggest the most frequent contact and address instead of the least probable one.
For instance : the colleague i write to and who writes to me 10 times a day, instead of the guy with the same first name that wrote once me 12 years ago. It really looks like the completion list is always sorted by frequency, increasing instead of frequency decreasing, this really drives me crazy.