However, Thunderbird seems to be stuck in a development limbo... I legitimately wish that the 'Mozilla Core' approach like they originally created Thunderbird from either needs to be done again from Firefox once proper multi-process via Electrolysis is enabled in mainline, or for them to simply clean-slate a new mail client.
I remain skeptical though, as there simply isn't the resources or interest to pour into Thunderbird as they had when it was first created.
Unfortunately, they instead prefer to devote those resources towards other projects for a couple of years, before then "giving up" on those projects and finding something new to pour their resources into.
With just one or two full-time developers, Thunderbird could be an absolutely awesome e-mail client. I already think it's pretty decent and I use it every day and, unless I'm not seeing something, I don't think there's any need to constantly introduce new features into it. E-mail doesn't really change all that much so it would mostly be maintenance and support, fixing bugs, etc., instead of constantly pumping out new versions with "killer features".
What would Thunderbird be like today if a fraction of the resources devoted to Personas, Hello, Firefox OS, and so on had instead been put towards Thunderbird?