Rated 5 out of 5 stars
I can get the toolbar to show up by following the instructions in a previous response " If the view switcher isn't in the toolbar when you enable it, then right-click on the toolbar, select "Customize...", and add the "Folder Views" tool from the palette"
However that comes up is the standard pull-down list which allows you to switch with 2 clicks. I cannot get the add-on to work as before.
UPDATE: The arrows are back in this beta release:
Give it a try!
As it says at https://addons.mozilla.org/en-US/thunderbird/addon/folder-pane-view-switcher/, the arrows are gone and at this time there is no way to get them back. The change was out of my control and I don't like it any more than you do.