Have absolutely NO idea what I did, but somehow they have returned of their own accord!!!

Open tabs still missing from previous session, but I can (hopefully) cope with that!

Have you made a backup of the system?

When yes you can restore the bookmarks.

Forgive me, but I am absolutely hopeless with computers. a) Do you mean backing up the entire operating system? & b) I wouldn't have a clue how to do it anyway!

What I have done was follow a solution on Google where I typed in 'about:config' then changed one of the settings from false to true. My bookmarks mysteriously re-appeared after that.
Now that they have returned I have also saved an HTML file in 'My Documents'  and will back that up to an external drive so if this ever happens again, I can (hopefully, if I can work out how) re-install my bookmarks from this file.
Thanks though for trying to assist.

Backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running.

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 try to restore the session stored in the most recent upgrade.jsonlz4-<build_id> file Copy this recent upgrade.jsonlz4-<build_id> file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file (make sure to backup the current sessionstore.jsonlz4).

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

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thanks for trying to assist, but as an 'oldie', all that was total gobbledegook to me! Sorry, I have absolutely no idea what any of that means or where or how to start in trying to follow it.

Hi devilishlygood, I'll say part of what cor-el said in a more step-by-step order. Hopefully that will help a bit.

(1) To open your profile folder...

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" button.

(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
• some old .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?

Looking Inside Compressed (.jsonlz4) Files

There isn't a convenient way to view the session details. I created a tool online to extract their contents. You can drag and drop a file onto the white box on the following page, then click the Scrounge URLs button:

https://www.jeffersonscher.com/ffu/scrounger.html

Can you identify a file that has what you need?

Wow jscher2000, I actually managed to follow all that. Believe me, that's impressive for me!.

There were 4 files in that folder.  I managed to 'scrounge' 2 of the files (previous.jsonlz4 & upgrade.jsonlz4-20190717172542) but as feared when I saw their date and times, they only opened tabs that I opened AFTER the Firefox upgrade. I think I screwed myself by running Ccleaner which would have wiped a lot of data when the update was taking place.
BTW, the other two files that were in that 'sessionstore-backups' folder both had the same file name (recovery.baklz4) and same time and date - both after all my troubles began. Nothing before unfortunately.
Although losing so many tabs that I hadn't bookmarked is a pain, I do at least have all my other bookmarks back, and if I had to choose one over the other then bookmarks would have been my chosen option.
Thanks for trying.
devilishlygood

You can check in Windows Explorer if there is a previous version of the sessionstore.jsonlz4 file available.

You can look at this utility to browse System Restore points.

• System Restore Explorer
  https://www.nicbedford.co.uk/software/systemrestoreexplorer/

In addition to checking in restore points, someone mentioned in another thread that they were able to use Recuva to undelete recently deleted recovery.jsonlz4 files. I've never tried that myself. https://www.ccleaner.com/recuva

Can you confirm that you still use the same profile?

What is the content on the times.json file in the profile folder if you check this in a plain text editor like Notepad or open the file in a Firefox tab and open the Raw Data tab?

What happened?? I was trying to look for (without success) the times.json file that you mentioned and found myself in 'about:profiles'.

I couldn't see much that I thought applied to me but clicked "Launch profile in new browser" and  guess what? That brought up a new window ......... WITH ALL MY OLD TABS OPEN!!! Brilliant, I now have everything I thought I had lost.
Thank you all for all of your assistance.