Thanks, but I done all this and have checked and rechecked

When I click History, Restore Previous Session does not appear as an option (not even is it greyed out)

Below Startup I have selected Open previous windows and tabs.

I always close Firefox with the tabs still open

In the Privacy & Security panel under the History section, the Firefox will drop-down menu is set to Remember history

Except on 2 or 3 occasions, my tabs have been restored and I have them bookmarked in any case so I haven't had to deal with the issue. Here is a link to a thread which may help.

https://support.mozilla.org/en-US/questions/1380313#answer-1513897

"Restore Previous Session" is missing or grayed, check either:

• "3-bar" menu button > History > Recently Closed Windows
• "Library" toolbar button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

Do those have anything to restore?

If not, do you notice any other history missing?

a Restore Previous Session option does not appear at all

There are 23 items under Recently Closed Tabs and 3 under Recently Closed Windows but these are unhelpful as they don't include all the tabs I have open, just the ones I have recently used

Terry Suggested I bookmark my tabs and also to check a thread which may help.

https://support.mozilla.org/en-US/questions/1380313#answer-1513897

I do have a folder of tabs to use in emergency but since my open tabs change each session it is pretty much impossible to do this every time I close Firefox and impossible if it crashes

I checked the link but its way beyond me to interpret!

What I really need is a permanent solution rather than a workaround...

There are 23 items under Recently Closed Tabs and 3 under Recently Closed Windows but these are unhelpful as they don't include all the tabs I have open, just the ones I have recently used

The Recently Closed Windows should have the last 3 windows you closed. At startup, those would be all from your previous session, but over time, they probably will be replaced with windows you closed in the current session.

Terry Suggested I bookmark my tabs and also to check a thread which may help. https://support.mozilla.org/en-US/questions/1380313#answer-1513897

That thread is how to recover tabs from an old file, which I can go into more, but to bookmark all the tabs in the current window, just press Ctrl+Shift+D and Firefox will pop up a dialog asking for a new folder name.

What I really need is a permanent solution rather than a workaround...

The puzzle is why it stopped working, since it is supposed to work. Possibly there is a disk access or data corruption problem. Have you done a normal shutdown and restart of Windows since this problem began? The purpose of that is to release file locks and clear any possibly corrupted memory.

Are the files in the sessionstore-backups folder being updated during a Firefox session ?

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 copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.jsonlz4 to replace the current file with Firefox closed.

• make sure to backup the current sessionstore.jsonlz4

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

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data