Splitting focus to create a mobile client while the desktop client all their remaining loyal users actually care about lies moribund is a great idea!
But it just isn't thinking far enough outside the box. What they really should do is create an entire Thunderbird OS based on web technologies and XUL, and sell it to mobile phone manufacturers! It could be huge!
I'm full of great ideas. Have they considered that email client interfaces are hard to understand for your grandparents? Maybe they could make it look like your living room, with each piece of email a book on a shelf. And they could have a cute little animatronic talking bluebird that you could ask for help-- call it, I dunno, want something non-threatening... Thunderbird Bob!
One more? Short but sweet-- Thunderbird Vista!
Really, the possibilities are endless.