Sorry to hear about this problem. First, please back up the recent files in the sessionstore-backups folder somewhere that Firefox won't overwrite them.

Next, do you have a series of files with names matching this pattern:

upgrade.jsonlz4-build_id

The latest I see on mine is

upgrade.jsonlz4-20250827004350

If you want to try using one of these files -- assuming they are more recent than what previous.jsonlz4 provided -- you can try the manual swap procedure:

(1) right-click > Rename sessionstore.jsonlz4 to sessionstoreOLD.jsonlz4

(2) right-click > Rename the sessionstore-backups folder to sessionstore-backupsOLD

(3) Copy in the file you want to restore, and rename it to sessionstore.jsonlz4:

For example, upgrade.jsonlz4-20250827004350 => sessionstore.jsonlz4

(4) Start Firefox and it should read the swapped in sessionstore file and restore whatever was in that file

Any improvement?

Also, you didn't mention any other problems, such as change to browser appearance, bookmarks, history, etc. To check for the possibility that Firefox started up in a different profile, you could also check the first section of the following article: Recover lost or missing Bookmarks.

Hi jscher2000 - thanks for your reply to help out!

Yes, I do have 3 upgrade files, but among them the one with the most recent date modified is the same date as the previous.jsonlz4 from last month (the session the tabs seemed to re-open to).

The browser appeared to be the same and open with the same account.

Any other method to try or recourse before scrolling through weeks of history to recover the lost tabs? (some of which will likely not show in history as I opened them using "Open in New Tab" and hadn't visited them yet, but saved for later)

I don't know why you don't have more recent files. Do you actually have the recent history?

Yes, I did see the recent tabs in my history when the browser had re-opened. I also find it odd that it has the history intact, but not the session tabs themselves. Any other method to try before scrolling through hours of history to find the relevant pages?

Sometimes there is a backup of a more recent files captured during a Windows update. This post has the steps to check for and export recovery.jsonlz4 (or sessionstore.jsonlz4) from one of those: https://support.mozilla.org/en-US/questions/1446921#answer-1652658

Thanks for that additional tip. Using ShadowExplorer I found one file from 8/27/25 (only about a week before the crash), but when exporting either the recovery.jsonlz4 using the Session History Scounger on your website it says "File could not be read?". I then exported the recovery.baklz4 which despite being the same size as the .jsonlz4 file (6,003 KB) and being larger than other files in the live sessionstore-backups folder (4474 KB - which showed a lot of urls just older), the .baklz4 file only showed 3 urls in the Unstructured URL List output. Any steps I'm missing here or is your tool not fully compatible with the exported files?

I don't know why you don't have more recent files. Do you actually have the recent history?

I've seen in Nightly where the update reset and erased some of the files in the sessionstore-backups folder. Made a complaint in the Nightly chat back in March and after that they introduced the sessionsotre-logs folder. see screenshot That's also when I started making copies of the sessionstore-backups folder. Nightly failed to restore my session today and I have a copy of the folder to show the difference. I asked about this issue today in the Nightly chat, but it went silent. see screenshot

Josh, I would suggest not only make copies of the folder, but also run more than 1 version of Firefox and if possible on more than 1 device. Then use sync so that your data is not in just 1 places. see old screenshot If you have no newer files than the month old ones, it sounds like you're out of luck.

Hi Josh, sometimes the recovered files have illegal characters or long runs of garbage. I don't have any other thoughts on where to find old files that would have a better track record. The "undelete" program Recuva usually gets worse results, but that would be the last thing to try. (https://www.ccleaner.com/recuva)