Hello. I am not familiar with that file; I have never seen a reference to an upgrade.* file. Is it on your desktop? If so, it may contain your old data. In the meantime, here are some articles and a detailed answer on restoring a session. https://support.mozilla.org/en-US/kb/how-do-i-restore-my-tabs-last-time https://support.mozilla.org/en-US/kb/how-restore-browsing-session-backup https://support.mozilla.org/en-US/questions/1378082#answer-1508025 https://support.mozilla.org/en-US/kb/restore-previous-session I suggest you consider backing up your profile. https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

upgrade.jsonlz4-<build_id> files are a backup created during an upgrade of Firefox and contain a backup of the session at the time Firefox updated to a new major version (100 > 101). You can look at this tool to inspect a compressed jsonlz4 sessionstore file. *https://www.jeffersonscher.com/res/scrounger.html