Oh jeeze the last thing Thunderbird needs is to be raked over the trendy UX coals the way Firefox has. If Chrome's market share has come at the expense of Firefox it may be in part because many people who jumped ship - myself included - found that each Firefox release was becoming successively more and more "chrome-like" without offering any of the benefits that make Chrome a compelling offering. In my case it was speed and performance on a 2006 Mac Mini running 10.6 - firefox was bloated slug that constantly screamed at me to upgrade my OS; Chrome ran as fast as it does on modern hardware and never complained about anything. Chrome's UI and core functionality haven't changed much since I started using it, either - I grew to dread Firefox updates as you never knew if it was going to pull an iTunes and reboot with some new horrible "feature" that didn't have extensions to revert the behavior back to prior functionality - Firefox deciding it was going to handle PDFs inline, and that functionality being far beyond slow and a real pain in the ass to disable - was the last straw for me. When I left the browser half of my extensions and customizations were to undo things the devs had "improved" over the years - the other half were ad and flash blocking extensions, which Chrome does almost as well.
TLDR; Firefox was awesome when it was Mozilla Without The Cruft. Then it started to cruft up and bloat up and horrible terrible very bad things started to happen to the UI and now it's Just Another Browser. Which is fine, really. Thunderbird does not need to be "innovated" in the same way - Firefox needs to be replaced by Firefox Without The Cruft the way Firefox replaced Mozilla. Maybe stick to the UNIX idea of "do one thing well" this time around, instead of "do one thing reasonably well and an increasingly lengthy list of perpedicular things in a totally half-assed fashion."
I used Netscape Navigator until IE5 (Mac) came along, then I used Mozilla until Safari popped up, then Firefox until it drove me to Chrome. Chrome Just Works on everything I run it on and has never nagged at me to update or screamed at me to upgrade my operating system Because Reasons. It has yet to roll out a game-changing UI element that I hate, and it isn't slowly modeling its overall UX to resemble the competition. I hope the Mozilla foundation keeps going because we need choice, now more than ever - and maybe one day they'll be my choice again.