Installed MozillaThunderbird-60.0-8.2.x86_64 from https://build.opensuse.org/package/show/mozilla/thunderbird60 on Tumbleweed. Restarted Thunderbird, opened an email with images that I'd previously whitelisted and it loaded them correctly. Opened an email with non-whitelisted images and it still blocked them. Seems to be good here.