It should have created an ''Old Firefox Data'' folder on your desktop, as this knowledge base article describes: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

You need to copy the sessionstore.jsonlz4 file from the "Old Firefox Data" folder on the desktop to the current profile folder. *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page. *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox