As of Thunderbird 48, the folder pane view-switching arrows have been removed from Thunderbird (previously, they were present but hidden by default, and this add-on un-hid them). They have been replaced with a new folder pane toolbar which displays immediately about the folder list which you can enable to get a drop-down menu view switcher. 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.

The remaining functionality of the add-on, i.e., automatically switching to the "All Folders" view when you over over the toolbar while dragging a message, remains in effect in Thunderbird 48+.

On older versions of Thunderbird, the view-switching arrows functionality is retained.

New French translation from Bigpapa.
Localize the install manifest so that the description of the add-on in the user interface will be translated (for the languages for which I have translations).

Add Dutch translation, thanks to markh from BabelZilla.
Add Simplified Chinese translation, thanks to yfdyh000 from BabelZilla.
Add icon.
Compatible up to thunderbird 15.0a1.

Reduce delay before switching views from two to one second. Two is too long.
Disable the automatic revert to the original folder view whenever any new message is copied into any message folder while the drag is in progress. This is no longer necessary in the current architecture of the add-on.
Fix a bug which would cause the original folder view to be lost if the user moved his/her mouse into the folder view title area for long enough for the view to be switched automatically, then moved the mouse out of and back into the folder view title area.

You now have to hover over the view title, i.e., the title to the left of the left/right view switcher arrows, to get the view to switch to “All Folders” during drag and drop. Previously, you could hover anywhere in the folder tree, but that is no longer the case.

This functional change is unfortunately necessary to work around some deficiencies in the drag & drop facilities provide by Thunderbird.