Quoting Andrew McGlashan (andrew.mcglashan@affinityvision.com.au):
I think that TB was "almost" orphaned, but it hasn't been; still it is not getting much real support from Mozilla whom seem to care only about their 57+ browser....
Firefox gets them revenue. Thunderbird doesn't. It's always important to remember who's the customer and what is the revenue model for any profit-oriented business. In this case, Mozilla Corporation makes its revenue primarily from Google, Inc for making Firefox & SeaMonkey's default search engine be Google Search, and from click-through revenlues on advertisements placed on search result pages. https://en.wikipedia.org/wiki/Mozilla_Corporation
TB is the absolute best email client and it really needs more love to keep it that way.
The absolute best e-mail client is mutt. The best _graphical_ e-mail client is mutt in an xterm. ;->