I'm testing the following patch which I think ought to work:
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RCS file: /cvsroot/pkgsrc/mail/thunderbird/PLIST,v
retrieving revision 1.64
diff -u -r1.64 PLIST
--- mail/thunderbird/PLIST 16 Dec 2018 08:12:15 -0000 1.64
+++ mail/thunderbird/PLIST 20 Dec 2018 21:25:17 -0000
@@ -2434,8 +2434,7 @@
lib/thunderbird/chrome/icons/default/default32.png
lib/thunderbird/chrome/icons/default/default48.png
lib/thunderbird/chrome/icons/default/default64.png
-lib/thunderbird/chrome/messenger/content/branding/about-background.png
-lib/thunderbird/chrome/messenger/content/branding/about-logo.png
+lib/thunderbird/chrome/messenger/content/branding/about-logo.svg
lib/thunderbird/chrome/messenger/content/branding/about-wordmark.svg
lib/thunderbird/chrome/messenger/content/branding/about.png
lib/thunderbird/chrome/messenger/content/branding/aboutDialog.css