“New tab” button shortcuts:

Ctrl+Click: opens new tab after current tab (or at the end, if you changed the default behavior to open after current tab)
Shift+Click: opens a new window
Ctrl+Shift+Click: opens a new private window (if allowed to)

Permissions used:

Access your data for all websites: to use containers.
Read and modify bookmarks: to detect when a tab has been dropped outside the sidebar (but not on the bookmark bar).
Read and modify browser settings: to use Firefox preferences and theme.
Access recently closed tabs: to reopen closed tabs.
Access browser tabs: to get URL, title and favicons of tabs.
Access browser activity during navigation: to trigger an extra animation when a page has finished loading its resources.