Steps to reproduce:
Download the Firefox or Chrome extension for are.na (https://chrome.google.com/webstore/detail/arena/lkihjlcipnbgeokmfnpogjfflofbfhga), try clickin on the button that is inside of the toolbar.
There should be a small popout window that allows you to use the interface presented by the extension.
Orion and macOS:
macOS 12.01 Orion 0.99.108