How do I save sessions (permanently) or sort of favorite them so that I can open the groups of tabs later on? Can we replicate Simple Tab Groups functionality with Tab Session Manager ? Thanks.

One of the most useful add-ons.

very useful tool for all MFF Users like me..

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!