Firefox stores session data in sessionstore.jsonlz4 in the Firefox profile folder. This file is only present when Firefox is closed. During the session (i.e. when Firefox is running) the sessionstore-backups folder is used. If Firefox restores an older session then there might be a problem with the sessionstore.jsonlz4 file or you might be switching to another profile. In case of a crash when sessionstore.jsonlz4 is missing might Firefox fallback to a file in the sessionstore-backups folder.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

You will normally find these files in the sessionstore-backups folder:

• previous.jsonlz4 (cleanBackup: copy of sessionstore.jsonlz4 from previous session that was loaded successfully)
• recovery.jsonlz4 (latest version of sessionstore.jsonlz4 written during runtime)
• recovery.baklz4 (previous version of sessionstore.jsonlz4 written during runtime)
• upgrade.jsonlz4-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file with Firefox closed.

• make sure to backup the current sessionstore.jsonlz4

Thank you for your response. Pardon me for pressing further...but, I am trying to understand (I have some other non-firefox issue that I am working on that is causing frequent crashes and I really would like to make sure a more current history is represented). I appreciate the help as I want to understand the process. Searches turn up how to "restore" a session, but not a lot on the process on "when" and "how often" session information is saved, which, since I am always restoring to "long ago" is of more interest to me.

I checked the profile folder as you indicated and see a recovery.jsonlz4 (and recovery.baklz4) with a timestamp of just a few minutes ago -- which is in fact more recent than even when I first checked as a result of you response (and Firefox had not been closed between these timestamps). So in my, admittedly, minimal understanding, Firefox is creating a restore point fairly frequently. Do you know how often Firefox does this? Is it the close that causes this information to be transferred to sessionstore.jsonlz4? At the moment, I have only a sessionstore.jsonlz4.tmp from a few days ago.

I understand your point about a problem with file corruption leading to restoration of an older session, which would likely correspond with the timestamp on previous.jsonlz4. Any thoughts on why, when Firefox seems to be making a new recovery.jsonlz4 file every 5 minutes or so, that they would constantly be corrupt and needing restoring from the previous.jsonlz4 file.

Again, thank you for your time and assistance.

You should never see temp files like sessionstore.jsonlz4.tmp or otherwise numbered sessionstore.jsonlz4 files then this can be a problem with security software that keeps an handle on this temp file and prevent Firefox from moving it to sessionstore.jsonlz4. Firefox updates recovery.jsonlz4 and recovery.baklz4 when necessary (i.e. session data like open tabs has been changed).