It is new and to do with 'Updates'. the icon is called a doorhanger and it usually means that an update is available. Some info for reading as it would appear they are still ironing out minor bugs. https://bugzilla.mozilla.org/show_bug.cgi?id=1578175 https://bugzilla.mozilla.org/show_bug.cgi?id=1549242

Thank you. Indeed, when I opened the "About Thunderbird" Help menu item there was a note that I should restart Thunderbird to install the pending update.

RFE: Provide a brief note ("Update Pending"?) when users hover over the arrow graphic?

Thank you 8- )