[j1elo]

No, I don't have muscle memory, and in fact you have introduced me to the Ctrl+Shift+B shortcut to show or hide the Bookmarks Toolbar! :-D I won't use it though, because I have it visible and it's not something one typically changes a lot.

With "side panels" I mean what strictly speaking Firefox calls "Sidebar": menu View -> Sidebar -> choose ONE among Bookmarks, History, Synced Tabs, Bookmark search plus 2, or Tree Style Tab.

Why the hell I cannot have e.g. Tree Style Tab AND a Bookmarks sidebars on the left, at the same time? Seems silly to me. Ages ago I worked with Qt and made desktop applications that could have detachable panels (QDockWidget), or their native equivalents such as palettes on Windows, that could be placed anywhere on a main window; but now that we're living in the future it seems we went backwards on what our UIs are able to do.