The Mint20 software updater just pushed a TB update from 68.1.0 to 78.7.1 and now, I too have a folder pane full of useless whitespace between folders. I've read what I believe to be every one of the threads on this problem in 78.3.x, but even after setting toolkit.legacyUserProfileCustomizations.stylesheets true, locating the current profile folder, and creating the /Chrome/userChrome.css file, ''*none*'' of the various .css hacks suggested for reversing this change or regression work in 78.7.1. All such changes are ignored. If anyone has a way of reversing this bug or regression in 78.7.1, I'd surely appreciate it.