The original order of the tabs is lost when the window is restored. In my case, the extension is to blame: [https://addons.mozilla.org/en-US/firefox/addon/tab_position_options] The error is in option: [New Tab][Right of current tab], It works if you leave it as [Default].
Another solution is to use https://addons.mozilla.org/en-US/firefox/addon/tab_position_options
