Are you certain that you are in the correct Library folder?
I'm not sure how else to say this ...
The Thunderbird folder won't be in the main Library, but you should see it in "your user folder"/ Library.
The path to that location will be ~/Library. You can type that path, as is, in the Finder Go menu, Go To Folder...
What happens when you:
From the finder, press Shift-Command-G
Replace whatever is in the box with ~/Library/Thunderbird
(You should see the contents of the Thunderbird folder)
How long have you been using Thunderbird?
Looking around in prefs, looks like you can do a few changes to fonts right there, in Thunderbird's preferences/Display, where you can select "Advanced" for more control over fonts. Maybe you can make the changes that you want there, and no need to manually make any changes.
Hope that helps!