This extension shows an overview of your upcoming calendar events in the Fastmail sidebar. Just install the extension, go to its options page, and add your calendars! The extension works with both light and dark themes, and your list of calendars is synced with your Firefox account, so any browser that you're logged into will have you calendar overview.
Fastmail changed their HTML; this update changes the name of the root into which fastmail renders so that it can detect the user's color scheme properly.
