If that CID Firefox version uses sessionstore.js or sessionstore.jsonlz4 then you can use this file directly.

You can copy a sessionstore file to the main profile folder and rename the file to sessionstore.jsonlz4 to replace the current file (make sure to backup the current sessionstore.jsonlz4). If you only have a sessionstore.js then make sure to remove the existing sessionstore.jsonlz4 and possibly also rename/remove the sessionstore-backups folder to make it possible to fallback to the legacy sessionstore.js file.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Thanks for your reply.

CID has sessionstore.js and sessionstore.jsonlz4 in its Roaming folder but I can only find sessionstore.js in Firefox.

I'm not clear on what you wanted me to do with the about:config page?

If you have a sessionstore.js file in Firefox then this is likely a leftover from an older Firefox version. Current Firefox releases use sessionstore.jsonlz4, but this file is only present when you close Firefox, so not seeing sessionstore.jsonlz4 would suggest that Firefox is running. If you have both sessionstore.js and sessionstore.jsonlz4 in CID then this would suggest that sessionstore.js in CID is also a leftover and sessionstore.jsonlz4 is current (you can check file dates).

To move tab session data from CID to Firefox you can copy sessionstore.jsonlz4 from the CID profile folder to the Firefox profile folder with CID and Firefox closed.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

Thank you very much. Mission accomplished! :)