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Tab Mix Plus is a fine add-on for Firefox but a bit of overkill for what I wanted. So I uninstalled it. Unfortunately, after that, Firefox’s built in session restore no longer worked.
It seems the TMP turns it off during the install (as TMP has its own session restore feature) but doesn’t restore it when uninstalled.
Correcting this is a case of running about:config in the address bar (this displays a list of Firefox configuration settings) and then finding the one named “browser.sessionstore.enabled”. This will be set to False. Right click on this and select “Toggle”. This will change False to True.
Voilà session restore is, erm, restored.