I don't think people want to change email addresses very often. How do I know Mozilla will still be doing this in 5-10 years? (Edit: Others have pointed out that, if we can bring our own domains, technical users can retain their address. However, for non-technical users that's not an option.)
Also, I'm not sure about charging a fee at the start (except for TB contributors) and providing a free tier later - reverse of the usual way of launching a product. Maybe this is a soft launch to shake out the bugs and build a little momentum, and you can pay if you want to take part?
Mozilla could do something awesome here. I hate to say it, but here is a chance to start fresh and make big, legacy-breaking changes to Thunderbird. The new audience - which should become the vast majority if they are successful - won't care if it's not like the old Thunderbird (possibly unlike many on HN). Here is a chance to do something special and the mail client is all most users see or understand.