For those who have lost tabs, I discovered that selecting Exit from the hamburger menu instead of X (close) avoided loss of tabs. Also, if X is clicked by mistake, I found that restarting my laptop (or possibly just closing Firefox and reopening it, I'm not sure) restored the tabs I had open.

When first opening Firefox, a succession of several sessions, one with only 1 tab and others with a group of tabs, rapidly appear before displaying just one session with the right group of tabs. I'm now going to disable this add-on and stick with Firefox's "open previous windows and tabs" option.

Edit: The behaviour above (2nd paragraph) still happens even after disabling the add-on, both with X (close) and Exit. AND this edit paragraph is missing (despite definitely clicking Update review) if closing with X!!

This extension is superb on a PC!
Pros:
* Saves tabs for easy access later
* Can import/export tabs for use on another device
* This extension doesn't scrape and sell your data AFIK

Cons:
* Currently on Android Firefox some features such as adding current tab or just one page at a time don't seem to be available, but I know this scenario is probably kind of rare.
* Firefox on Android currently doesn't let OneTab save inactive tabs, so you have to open those tabs to make them active and then save to OneTab

I love the pros enough that I'm ok with the cons and can just work around them.

very useful

Amazingly easy to see and quickly delete tabs that I'm not using anymore. A godsend for a tab hoarder like me. Also makes it easy to copy a list of URLs from a particular window!

Mod strongly discouraged.
I usually set my windows or tabs using firefox bookmarks and it works fine.
But Onetab when it opens them again the tabs are all UPDATED AT THE SAME TIME!!!!
As a result, I reopened a window with 70 tabs and they were all updating 1 by 1 and loading all 70 at the same time, instead of doing what bookmarks do and putting them on hold.
So it took 20 m and crashed.
Fortunately, I was able to get other windows, the one it hadn't tried to open or the one where I'd put a lock... but when I relaunched the one that was loading, I had 6 tabs out of 70 ... lost forever ...

It's convenient when it works. Today is the THIRD time it has lost all my bookmarks. This time was without warning because I had not done anything that should have changed any settings.

If you do use this add-on, be sure that you are frequently exporting your list so that you reduce the amount of bookmarks you lose between OneTab failures.