Hello Thomas, correct, this is the default behavior of FF -> https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/sidebar_action

This is a security default, so that an install doesn't go unnoticed by the user.

Note that:
- If you immediately redisplay the previous sidebar you had, then subsequent new windows use the one you just selected.
- When you close FF with a sidebar open, next time it restarts, it reopens with the same sidebar you had at time of close.
These are FF behaviors at this time which an add-on cannot modify.