I use ff. Tried vivaldi but its not opensource and uses the same engine as chrome. And i havent found anything even close for what firefox offers. It fits my needs perfectly.
Thunderbird as far as i know is abandoned by mozilla.
As for windows i cant say much because i dont use it for many years.
There isn't many alternatives out there. I'm also not following the current state of afairs with mozilla. I know they are more end more working together with closed source projects as far as integration goes (pocket and all) but apart from that i haven't hear more. @deavmi do you have some links in hand about it?