Do we need an extension ? Rated 3 out of 5 stars
The only thing this extension does, which can not be done without, is keep the folder pane on the left. Is that really more useful than just having the message cover the whole area ?
Stock Thunderbird includes "open in new tab", with shortcut Ctrl-O, which show the message covering full area (no message list, no folder list). Usual shortcuts (like press f for next message, n for next unread message) work as expected in this view.
Simple. No extension, no incompatibility, no trouble.