You may have corrupt sessionstore file(s).

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the sessionstore file(s).. Then rename or delete it. Restart Firefox.

There was no sessionstore.js file within the profile; rather, there was a sessionstore.bak file and a sessionstore.bak-20140923175406 file, neither of which had been updated since 2014. There was also a sessionstore-backups folder, which had recovery.js and .bak files, a previous.js file, and three upgrade.js-[date] files. I removed both sessionstore bak files and the folder, restarted Firefox, and now Session Manager works. There is also a folder with a copy of the profile from 2012, and that has a sessionstore.js file, but I did not remove that, since that's just an old copy of the profile. It appears that the cause was the recovery.js and/or .bak files in the sessionstore-backups folder, since those were the only files that had recent modified dates. Thanks for the suggestion to look for sessionstore files -- that was not the answer, but it led me to the sessionstore folder, which had the apparently bad files.

Glad to help. Safe Surfing.

The same thing happened again, within a couple more Firefox restarts. OK, I'll remove the latest recovery.js and .bak files, but now I need to know how to prevent the problem from continually recurring. Session Manager is almost useless if it won't store crashed sessions or any other sessions I want to store.

I now have some hint as to what happens as the problem recurs. I'm working in Session Manager in Save mode, and when I enter a Name, the Save Session is not greyed out. Then, I switch to "Load", and I look to see what windows and tabs various saved sessions contain. All of a sudden, sessions having multiple tabs in multiple windows, which I was able to display (and load) just a few minutes earlier, will no longer display, and instead I get the message "This operation failed due to a file access error: unknown error". I cannot then load those sessions, only single-window sessions. After that, if I switch back to "Save" mode, I cannot save the current session, the Save Session is greyed out.

So, the question is, why is Session Manager failing to access prior sessions that it accessed earlier in the same Firefox session, and why is this making it no longer possible to save the current session or any crashed session.

I've called the big guys to help you. Good luck.

I am having similar problems. What do we do?