I just “upgraded” from version 68 to the latest version 78. My folders now have a black outline with a white interior instead of the yellow folder I did have before.
I also have several duplicate email address’s.
Tabs are on top right under File | Edit | View etc
Here are the release notes.
https://www.thunderbird.net/en-US/thunderbird/78.0/whatsnew/?locale=en-US&version=78.4.0&channel=release&os=%OS%&buildid=%APPBUILDID%&oldversion=68.12.1