I broke this in bug 1592739 and forgot to file a bug earlier.

On macOS, the sidebar in the Thunderbird window and the Thunderbird address book window are no longer vibrant - that means they now have an opaque background color, rather than being transparent with a glass-like appearance (showing blurred content from behind the window).

To fix this, all background-colors from elements behind the sidebar elements need to be removed (or moved elsewhere).

• In the main window, #messengerBox is an ancestor of #folderTree and has background-color: -moz-Dialog. This background color needs to be removed.
• In the address book window, it's #abContent which has background-color: -moz-Dialog. This background-color should be moved to #abResultsBox, #CardViewOuterBox or #CardViewBox.

This fixes the vibrancy. Like FX now when a simple LW-theme is active no vibrancy is used. This is for me a regression. The Mac chat tab needed some more tweaks.

Alessandro, with a simple LW-theme I gave the Mac sidebars the -moz-OddTreeRow. Is this okay?

Pushed by mozilla@jorgk.com:
https://hg.mozilla.org/comm-central/rev/aeb3be1b2053
Remove the background color from #messengerBox and #abContent under macOS to make trees vibrant again. r=aleca