fyi, addons can only be pinned (to the toolbar) if the addon developer has registered a toolbar button action, if not they are only listed in the extension button popup in a grayed out, disabled state. - This has as far as i know always bin this way and chrome and firefox behave the same way.
BUT there is one unfortunat decision issue here.
Addons can disable their toolbar button programmatically themself. In that case the extension button popup entry looks exactly the same as if it had no action registered but with the difference that the user can still pin the addon to the toolbar (via the right click context menu)
Toolbar action registered but action has been disabled, button is still pinnable
No toolbar action registered, no Pin to Toolbar context entry.