Open Adjacent New Tab от Pranav Mangal

This addon provides a keyboard shortcut to open a new tab adjacent to the current tab. It allows you to continue using the default shortcut (Control+T) to open a new tab at the end of the tab list and use a different shortcut when you wish to open a new tab next to the current tab.

By default the shortcut to open an adjacent new tab is Control+Alt+T.
This can be customized via Manage Addons -> Manage Extention Shortcuts.

Alternatively, you can click the extension icon to open an adjacent new tab.

The source code is available at:
https://github.com/pranavmangal/firefox-open-adjacent-new-tab