Sorry to hear about this problem. It sounds as though your latest session history file is corrupted. Could you start off by backing up the session files you have right now? Here's how:

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

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

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

In your profile folder, scroll down and double-click into the sessionstore-backups folder. Save all files here to a safe location, such as your Desktop or Documents folder. If not too much time has passed, we may be able to use them to recover your lost tabs.

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

• recovery.js/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.bak/baklz4: a backup copy of recovery.js
• previous.js//jsonlz4: the windows and tabs in your last Firefox session
• upgrade.js/jsonlz4-build_id: the windows and tabs in the Firefox session that was live at the time of your last update

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?

Windows Users: By default, Windows hides the .js extension. To ensure that you are looking at the files I mentioned, you may want to turn off that feature. This article has the steps: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/.

To view the contents of a session history file, you can drag and drop it onto this tool: https://www.jeffersonscher.com/res/scrounger.html (that's on my website, so if it doesn't work, let me know).

Do any of the files seem to have the missing tab addresses?

I reread the info a couple of times, and I didn't see where you said what to do with the files in that folder, other than to COPY somewhere, which would leave them in the folder. :-) I'm not worried about what might be in those files, though I did copy to another folder and then deleted all the files and restarted ff. I also set ff to restore previous session, so between those two I'll see how it goes.

I suggest using the "Scrounger" tool to see which recent file has the most complete data. Then if you find a file you want to restore, you can try the following:

Note: Not yet tested in Firefox 56

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 > "?" button > 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 "Show 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
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup -- these changes should occur:

• recovery.jsonlz4 is moved from the sessionstore-backups folder to the main level of the profile folder and renamed to sessionstore.jsonlz4
• recovery.baklz4 is deleted from sessionstore-backups

(3) Once Firefox has stopped updating things, while viewing the main level of the profile folder, rename sessionstore.jsonlz4 to OLDsessionstore.jsonlz4

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

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

• Uncompressed .js file => sessionstore.js
• Compressed file => sessionstore.jsonlz4

(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?

Still having problems with history restore tabs. Generally I have two windows open with multiple tabs in each. I also have FF set to "restore when starting" Now I can get one window to have the restored tabs (mostly), but the other is all tabs with no pages associated. Thx.

It seems your session history is getting corrupted somehow. I don't know whether this is related to something you have open or is caused by some other factor. I don't recall seeing any threads about this problem that identified a specific culprit.

Even when I delete all the session history files, open and start opening more tabs, I still don't get full tab restore. I guess I can hope that a future update will fix the problem. Thx.