Thunderbird on A Chromebook -- KIND A.....
I'm not aware of a simple direct way to install the Thunderbird client onto a Chromebook.....however.... it is possible to configure a Chromebook to run Ubuntu Linux side by side with the Chrome Operating System.
I did this, (installed Ubuntu onto my HP Chromebook 14) so that I could run Kingsoft Office for Linux. Honestly, it wasn't as hard as you may think but it was a bit time consuming. The entire procedure took me over an hour.. but in my case it was worth it.
So that's what I'd suggest you do, set up your Chromebook to run Ubuntu then proceed to load Thunderbird onto the installed Ubuntu OS.
Hope this helps ???