Could you check whether either of these are available (not hidden or grayed out):

• "3-bar" menu button > Restore Previous Session
• (menu bar) History > Restore Previous Session

If they are missing or grayed, check either:

• "Library" toolbar button > History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)
• (menu bar) History > Recently Closed Windows (and within each restored window, Recently Closed Tabs)

If that doesn't help:

Could you start by making a backup of your existing session history files? Here's how:

Do not exit Firefox, or if you closed it, don't re-open it.

(1) To open your profile folder...

If Firefox is still running:

You can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Open Folder" (or "Show in Finder") button.

If Firefox is closed:

Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer:

%APPDATA%\Mozilla\Firefox\Profiles

In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated.

(2) Copy out session history files

In your profile folder, double-click into the sessionstore-backups folder. Save all files here to a safe location such as your Documents folder.

(3) What files did you find?

The kinds of files you may find among your sessionstore files are:

• recovery.jsonlz4: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.baklz4: a backup copy of recovery.jsonlz4
• previous.jsonlz4: the windows and tabs in your last Firefox session
• upgrade.jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update
• various .js files from Firefox 55 or earlier

Could you take a look at what you have and the date/time of the various files to see whether you think any of them would have the missing tabs?

To preview the contents of a file, you can drag and drop it onto this page, then click Scrounge URLs: https://www.jeffersonscher.com/ffu/scrounger.html

That tool is on my site, so please let me know if it doesn't work for you.

Thank you for the reply. The most recent file from that folder that would have the missing tabs is from October 3rd, or over two weeks ago and it's called "upgrade.jsonlz4". The "previous.jsonlz4" file is from today, however it doesn't have any of the tabs in it. When Firefox decided to act up was when I was looking into an issue with a website that we use at our organization and I had a problem logging into it. Restarted Firefox a couple of times trying to troubleshoot it. I think that apparently rewrote the local copy of the "previous.jsonlz4" file. The issue turned out to be a local DNS server malfunction which I corrected. Does "Synced tabs" feature keep a copy of opened tabs? Because if I close a browser I can still access its opened tabs on another computer?

Yes, each time you open Firefox without restoring your previous session, the earlier previous.js is lost.

I don't know how quickly the latest local tab changes are recorded in Sync; I only set up Sync for bookmarks when I was testing it.

If your windows has updated since your last Firefox update, you might have one or more session history files in a restore point. DO NOT RUN SYSTEM RESTORE. Instead, use a program that allows you to export individual files from a restore point, in case you find a promising candidate.

Accessing Shadow Copies on Windows 8, 8.1, and 10

(1) Set Windows to show hidden files and folders:

https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files

(2) Download and run one of these programs:

• ShadowExplorer: http://www.shadowexplorer.com/
• System Restore Explorer: http://nicbedford.co.uk/software/systemrestoreexplorer/

Within the most recent restore point shown in the program, you can explore along this path to see whether you can find a recent shadow copy of your profile:

\Users\your-user-name\AppData\Roaming\Mozilla\Firefox\Profiles\

(3) Within the seemingly most recently updated profile folder, double-click into sessionstore-backups and Export any recent files you want to check to a convenient location such as your currently live desktop (not the desktop inside the restore point).

Then you can inspect the contents of that file using the Scrounger tool.

Anything promising?

Accessing Shadow Copies on Windows 7

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

Double-click into sessionstore-backups and right-click recovery.jsonlz4 > Properties, then click the Previous Versions tab

Windows should query restore points for older copies of this file, and you can use Copy... to back up the previous one to a safe location such as your Documents folder or Desktop.

(You also could check previous.jsonlz4 and other files.)

Then you can inspect the contents of that file using the Scrounger tool.

Anything promising?

Unfortunately this computer is a VM with a small virtual disk and restore space usage was set at 0. So no system restore file. I scrounged the most recent upgrade.jsonlz4 file from 10/3 and it seems to have most of the tabs. How do I bring them back into Firefox?

Hi draito, using the clickable links in the web page version would be simplest. Otherwise:

To try swapping in an older session history file so Firefox will load it, the standard "manual swap" method is as follows (assuming Firefox is still running):

(1) From inside Firefox, open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Open Folder" button. This should launch a Windows Explorer window listing the various files and folders in your profile.

(2) Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button Exit
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup -- a file named sessionstore.jsonlz4 should appear in the folder.

(3) Once Firefox has stopped updating things, while viewing the main level of the profile folder, right-click > Rename sessionstore.jsonlz4 to sessionstoreOLD.jsonlz4

If you see an older sessionstore.js or sessionstore.bak file, move it somewhere else or delete it.

(4) Copy in the file you want to restore, and rename it with the name appropriate for its format:

• Compressed file such as recovery.jsonlz4 => sessionstore.jsonlz4
• Uncompressed file such as recovery.js => sessionstore.js

(5) Start Firefox and it should read the swapped in sessionstore file and restore whatever was in the substitute file; if you do not have Firefox set to restore your previous windows and tabs automatically, use History > Restore Previous Session

Any luck?

Thank you! This was very helpful. I copied and renamed the upgrade.jsonlz4 file. The tabs are back with only the most recent ones missing. Question here: I thought the synced tabs feature would allow me to recover from such a situation but apparently it doesn't maintain a history. What else can I do to prevent that from happening in the future?

I don't personally use Sync for anything but bookmarks, so I don't know how it normally responds in that situation (where you loaded Firefox without successfully restoring the previous session). Possibly the data on the Sync server is updated to the new state immediately providing without an opportunity to tap into the previous state.

Thank again! My issue is solved for now.