hello, please go to the firefox menu ≡ > help ? > troubleshooting information, click on profile folder/show folder. a windows explorer window should open up - in there look for files starting with sessionstore which have a change date before the last restart. if you find any please immediately create a backup of them outside of your profile folder and report back...

Found a week's old, saved as 'sessionstore.bak-20140917194002'. The other related items, do not have any date, ie.: sessionstore.bak sessionstore-1.js

I have made a copy (?) on desktop..

Many thanks for taking the time!

So how should I treat the file now?

please try to open the file with a text editor and then if you can restore the session then when you copy the contents of the sessionstore.bak file(s) into this web service: https://firefox-session-restore.herokuapp.com/

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'An error occurred, please reload the page and try again'.

Did it 3 times..

Does this also happen if you open the file in the Scratchpad (Firefox/Tools > Web Developer)?

Click the "Pretty Print" button to format the file

Unfortunately yes. Same message! An error occured, please reload the page and try again.

Can't you open the file at all in the Scratchpad or does only clicking the Pretty Print button give an error?

Sounds that the file is corrupted and can't be used directly to restore the session. You would have to use other ways to extract the URLs from the file.