Since Supernova is effectively a new product, I thought I'd share a few points which are important to me. The new UI does look familiar, yet tidier.
I just installed the Beta
And notice all Owl support (Microsoft Office connectivity) is missing, which is also my excuse for using S/MIME, which we're not allowed to “not use” at work. I assume this will be addressed in future releases, as it's quite essential to those of us who use Thunderbird both at home and work. Thunderbird needs to support an MS Office centric environment for email, multiple calendars; me, coworkers, holiday..., address book (GAL), personal... with security features.
There is also the issue of memory gluttony. I suspect this is a feature of Firefox, as it shares this issue. Start it up, let it sit for a day or week, and suddenly it's using 8 GB - 16 GB of memory, i.e., nearly all the machine memory resources in a machine equipped with lots of memory for development. I can “cure” this behavior by regularly shutting down Firefox and Thunderbird, then restarting. Sounds simple, but isn't when this requires reestablishing multiple 2-factor authentication logins by phone and URL. This can waste 15 minutes or more for a restart just to reclaim memory consumed by these two Mozilla products. Don't do this, and eventually the machine becomes non-responsive because it is swapping huge amounts of gluttonized (probably used for caching, but won't rule out leaks) memory to disk. As a general rule, I'd say if memory hasn't been touched for a few hours, especially if it is used for caching, it should be released. This will improve long term performance and allow me to stay logged in to all those 2-factor authentication areas, many needed for work.
The UI looks great, glad to hear Thunderbird is getting a makeover, inside and out. Memory usage is an issue which has been brought up for more than a decade. Poor memory performance gets addressed, then quickly returns. Why not start a bunch of VM's to contain Thunderbird and Firefox, let them sit for days logged into various services; Google Mail, Facebook, Youtube, Microsoft SharePoint, then investigate to figure out what is causing all the memory demand? Then try to address the memory issue.