You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder

Linux: Under the page logo on the left side, you will see Application Basics. Under this find Profile Directory. To its right press the button Open Directory.

Locate the above file. Then rename or delete it. Restart Firefox.

Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

At startup, Firefox should read sessionstore.jsonlz4 and use that to restore your session, and then Firefox should remove that file.

During your session, Firefox constantly updates recovery.jsonlz/recovery.baklz4 in the sessionstore-backups folder.

Then at shutdown, Firefox will create a new sessionstore.jsonlz4 file at the main level of the profile folder and toss out recovery.jsonlz4/recovery.baklz4.

Q: Is Firefox removing your manually inserted sessionstore.jsonlz4 file after restoring your session? If not, you could delete it yourself and see whether that has been blocking Firefox from writing a new file at shutdown.

Does it help if you set Firefox to restore the session on the next start?

• Settings -> General -> Startup: "When Firefox starts" -> "Show my windows and tabs from last time"

Thanks for the tips everyone. Based on the details in jscher2000's reply I determined that at some point after copying the files over, Firefox severed whatever association it had between sessionstore.json4 and recovery.json4, and was only willing to load from the sessionstore or not at all. I suspect the first time around I deleted the recovery file when I copied the over sessionstore thinking it would consider a 'blank slate' as what it wants to recover.

There may be a simpler solution, but based on that I simply uninstalled firefox, reinstalled it, then copied over the new sessionstore.json4 file that firefox created with my old one, and hit 'restore previous session'. Then everything came back, and when I exited firefox using the 'close' function, it updated my tab changes.

I appreciate the speedy replies!