This seems to be related to how you close Firefox:

• Exit from the menu (Firefox menu > Exit, or File menu > Exit): all windows preserved, still can undo additional closed windows from the History menu

• Shut down Firefox using right-click on Firefox button on the Windows Task Bar > Close all windows: only last 3 windows restored by default*

The number that reopen in the second scenario appears to track the hidden preference browser.sessionstore.max_windows_undo which normally applies to windows you manually close during your session.

Please see this similar thread for how to modify that preference: Problems with windows in FF.

Does that work for you?

* When I try this, only the last closed window re-opens automatically.

Thanks for the help. It has definitely improved things. There was something else going on with bad behaviour of the networked home drives, so it's not purely a Firefox issue. I have never seen it on my own PCs where my home drives are on local disks. Anyway the solution certainly improved things.

Thanks for your help.