Um,... Thunderbird hasn’t been Mozilla for a number of years.
Independent, funded by contributions.
I’m still on Firefox, but it’s an old, old revn. that runs on top of XP-Pro. Mozilla hasn’t updated it in years, so any chicanery they pull today or going forward will pass me by.
It, Thunderbird, is owned by a subsidiary of the Mozilla foundation.
You might want to try palemoon.org. It’s not safe to run an old browser. palemoon is very customizable and it’s based on an old version of firefox but tweaked and updated.
#38 & 39
Modify Thunderbird to make the text larger by default and the icons better. Their updates of late made all the text too small and they went with the flat plain look for the icons.
Scroll half way down for directions:
Thunderbird email program
http://www.davidshelpsite.com/firefox/firefox_tabs.html
BTW you can use the scroll wheel on the mouse to make the text bigger and it will stay at the size you want for all messages until you close the program.