Features The features described here will only work on YouTube, Piped (default is piped.video), Invidious (default is yewtu.be), and Chat Replay.

Right click a web page while watching a video to reveal a context menu that allows you to; open the video in an alternative front-end while replacing the current tab, or open the video in an alternative front-end while creating a new tab.
Right click a video link (e.g. YouTube thumbnail) to choose an alternative front-end to open the video in (creates a new tab).

The Piped and Invidious instances can be configured about:addons -> YouTube Alternative Switch -> Preferences. Keyboard Shortcuts The keyboard shortcuts will only work while watching a video on YouTube, Piped, Invidious, or Chat Replay. Create New Tab:

Ctrl+Alt+Y - Open Current Tab in New YouTube Tab.
Ctrl+Alt+P - Open Current Tab in New Piped Tab.
Ctrl+Alt+I - Open Current Tab in New Invidious Tab.
Ctrl+Alt+C - Open Current Tab in New Chat Replay Tab.

Replace Current Tab:

Shift+Alt+Y - Switch Current Tab to YouTube.
Shift+Alt+P - Switch Current Tab to Piped.
Shift+Alt+I - Switch Current Tab to Invidious.
Shift+Alt+C - Switch Current Tab to Chat Replay.

The keyboard shortcuts can be changed in about:addons -> Manage Extension Shortcuts.

Developer response