Update: Firefox 70 and above is crap, does not load many tabs, the problem is not related to MySessions:
https://www.bleepingcomputer.com/news/software/mozilla-provides-workaround-for-firefox-70-not-loading-sites/
I would give this 3 stars (for decent operation, after all it is one of the few workable solutions to kind of replace the really good Session Manager from older Firefox). I was using this problem-free on Windows 7 64-bit at work. I still use this perfectly well at home on Debian GNU/Linux 64-bit (debian sid).
Then, 2 weeks ago I migrated my work Windows 7 machine to Windows 10, and also migrated my profile.
I had refreshed Firefox before the migration, so I had to install MySessions into the new profile under Windows 10.
What happens is that very soon, within minutes to an hour, MySessions stops Firefox from loading, refreshing, saving, or otherwise doing anything with its tabs. I need to disable MySessions add-on, reboot Firefox, can then work as usual, but need to re-enable MySessions if I want to see a tab labelled as "Replaced" (cannot easily load saved content of tab if MySessions is disabled).
I did not find any issue related to this, but if it helps MySessions is the only add-on I have installed.
On Windows 10 we use Micro Trends anti-virus, whereas on Windows 7 the company had been utilizing Symantec Anti-Virus software.
Since it has become unusable after migration, I am unsure if this is related to an interoperability issue with Windows 10, anti-virus software, or to migration issues.

The main problem with this extension for me is that it messes the bookmark search. My search results are now littered with a large number of repetitive session bookmarks, and it has become more difficult to find out if I have already bookmarked a web page or not. Others have also presented the same criticism, and the developer of MySessions has replied this way:

- "In the description of the extension it is written that sessions are saved in bookmarks."

and:

- "I recommend that you read the description before installing the extensions."

My answer is that the description doesn't say "the extension messes your bookmark search". It's not something that a user can deduce from the description, as the users mostly aren't developers. Even a developer, at least if he isn't a Firefox developer, can't know that there isn't a way to exclude session bookmarks from the search (I assume there isn't because it hasn't been done). Because of that I think the problem should be clearly explained in the description of the add-on.

The other problems that I have experienced are different bugs that in varying degree take away from convenience of using MySessions.

Despite of the criticism above I think it's very good that MySessions is available, and I raise my hat to CromS for developing it. He seems to be active in making the extension better and is listening to users when deciding what to do next (the search problem probably isn't going to go away). MySessions does the basic session managing, and to my knowing without this add-on there wouldn't be a usable session manager for Firefox at all as the old Session Manager can't be used in new Firefox versions.

Anyway, I will probably stop using MySessions as soon as another add-on with a design that doesn't cause search problems becomes available. A well functioning bookmark search is very important to me.