I have loved Thunderbird for many years - for several reasons -
multi platform
familiarity (understand comments about new innovation)
and overwhelmingly - the add ons (by far the best productivity for me is QuickText -http://extensions.hesslow.se/extension/4/Quicktext/ - a huge time-saver.
If TB does flounder - I will go with what ever client gets a port of QuickText)

I bet most that are worried are expressing a potential personal loss - and would like to say to Mozilla - thankyou for the years you have provided a free and awesome product to me.