Why do software people have to break something and call it an improvement? I hate the new font and line spacing in Thunderbird! The font is too small and if I use the keyboard to enlarge it, the spacing between lines makes the list of emails unusable. I am 81 years old. I started using the internet when it was Arpanet and text only. I was so happy when Mozilla came out with a browser. I have been happy with Firefox since the day it came out UNTIL NOW! I totally dislike the new font style too.
Put the original font and font size back in Thunderbird or at least make it an option. I cannot use it like it is today. As the saying goes, "if it ain't broke, don't fix it!"