I cannot find anyway to pin an extension to sidebar. In Firefox, extensions can be pinned to the menu bar. This is needed because some extensions provide quick access that effect the page/browser with instant click.
For example, Dark Reader extension, which enforces dark mode on page contents, provides the ability to quickly be switched off (page content will be back to light mode), and switched back on (page content dark-moded). Switching dark mode off instantly by having the extension pinned to the sidebar can be very useful the the dark mode is not rendered properly in certain web pages.