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.