This is in Australis. When using the Pocket addon https://addons.mozilla.org/en-US/firefox/addon/read-it-later/ I can customize Firefox to move the pocket toolbar control into the Customize and Control window. This control includes a button and a dropdown. Once in the window it acts weird. The drop-down arrow shows up between buttons, and then when selected the window which would normally pop up shows up in a seemingly random part of the screen (though I guess it's attached where the button is, but the button disappears).

Someone will need to investigate this to see if it is an issue with the Pocket add-on or with the Australis changes. If someone has a contact at Pocket they may be able to get to it sooner.

Forward duping to a bug with much more detail