Restore previous session after closing Firefox windows and leaving another window open:
– close Firefox and/or don’t open if you just find that you’re in trouble
– go to %AppData%\Mozilla\Firefox\Profiles\YOUR-PROFILE
– find sessionstore.jsonlz4 and rename if to sessionstore.jsonlz4-old
– go to %AppData%\Mozilla\Firefox\Profiles\YOUR-PROFILE\
– copy all files from there to some temporary directoy
– copy one of those files to %AppData%\Mozilla\Firefox\Profiles\YOUR-PROFILE
– rename that copied file to sessionstore.jsonlz4
– try to open Firefox and see if your tabs are restored
– if not or not all, try another file
There is a good tool to recover information from the .jsonlz4:
https://github.com/avih/dejsonlz4