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

最新版6.8.3だと、セッションを開きなおした時に保存した順番とは異なる順に展開されます。
保存時点でアクティブなウインドウのアクティブなタブだけ先（右）になるっぽい
具体的には一旦複数のタブを開いてセッションを保存、ブラウザ落としてセッション開いていくつかアクティブにしてもう一度保存、再度セッションを閉じて開き直すと再現できるはずです
12345の5タブ保存してあるとして1（今見ている）23（アクティブ）45（保存したのを開いたまま）でセッション保存して、ブラウザ落として開き直すと1（アクティブ）4523 として開かれます

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!

I don't recommend this extension. There are many session managers. I've used about 10 or so over the years. This is probably the worst UI I've yet seen.

The main drop down window of this add-on is disorganized and un-intuitive.

Where does it save the backups? In the preferences, it states, "Specify the folder to save the backup file. The save location is limited to the folder under browser download folder."

Does anyone have any idea what "the folder under browser download folder" means? I sure don't. I wonder if it's implicitly assuming the user is running Windows, but I really don't have the faintest idea. I looked everywhere I could think of, and did a search of my hard drive, but found nothing.

I'm very sorry I wasted time on this. Don't make the same mistake I did.