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

with the other extension

https://addons.mozilla.org/pt-BR/firefox/addon/always-right

to the developers i just want to thank you for such a wonderful add-on, 5 stars.

This is the Tab add-on I have been missing from Firefox. It allows you to save a session full of Tabs, and collections of Tabs - such as a Work collection or Study collection. This add-on is now integral to my workflow. Thank you, @sienori!

This add-on is AMAZING!!! It has saved me multiple times when my computer has crashed and when my wife's PC reboots randomly! It works flawlessly with Firefox, Brave and Chrome!

I love this add-on so much, I even donated money and would do it again and again!!

Great app. Makes organizing and finding multiple open sites easy. Can save multiple tabs as sessions for those sites you use often.