Rated 4 out of 5 stars

code pour l'extension custom Buttons (fleche droite)
/* CODE ok TB52-J2m06@laposte.net */
/* http://j2m-06.pagesperso-orange.fr/ */

var currentMode = gFolderTreeView.baseMode();
if (currentMode === "all")
gFolderTreeView.toggleMode("smart");
else
gFolderTreeView.toggleMode("all");

if (currentMode === "smart")
gFolderTreeView.toggleMode("unread");

if (currentMode === "unread")
gFolderTreeView.toggleMode("favorite");

/**************************************]
Cde pour flèche gauche
/* CODE ok TB52-J2m06@laposte.net */
/*http://j2m-06.pagesperso-orange.fr/ */

var currentMode = gFolderTreeView.baseMode();
if (currentMode === "smart")
gFolderTreeView.toggleMode("all");
else
gFolderTreeView.toggleMode("smart");

This review is for a previous version of the add-on (1.11).  This user has a previous review of this add-on.

That's easier than I thought it would be. ;-) I'll try to find some time in the next few weeks to add the buttons back to Folder Pane View Switcher.