Could you check the History menu, either:

• classic menu bar (tap Alt to activate) History > Restore Previous Session
• "3-bar" menu button > History > Restore Previous Session

If that is grayed out/unavailable, either (A) Firefox already restored the file, or (B) the file was missing or could not be used.

If Firefox restored the file, these menus many be helpful:

• classic menu bar History > Recently Closed Windows
• classic menu bar History > Recently Closed Tabs (separate list in each window)

Anything useful there?

Normally Firefox will retain session history for restoration unless one of the following clears it:

• tabs were loaded in private windows (either on demand or automatic private browsing / Firefox will Never remember history)
• Firefox is set to clear browsing/download history at shutdown
• a privacy-related extension
• an external cleaning or privacy program (e.g., CCleaner)

If your Firefox consistently fails to restore your previous session, these need to be investigated.

Firefox is set to remember all tabs and sessions. Previously I have had no trouble with this command in firefox. CCleaner has been running for ages on this machine. This problem has only occured since XP was reloaded.

espee8800 said

CCleaner has been running for ages on this machine.

Firefox's session history files changed format in Firefox 56 so either an update is needed to avoid session file confusion or setting CCleaner not to touch Firefox session data.

Thanks for your suggestions, I eventually found that an upgrade to CCleaner caused most of my settings to change including the session restore bit. Didn't realise that it was CCleaner problem and not a Firefox problem. Working OK now.