Clean boot and (apparently) shutdown this morning. My suspicion now points to a FF memory leak maxing out the swap file over time, preventing a clean shutdown or (eventually) causing a hard crash. xp appears to be unaffected and has carried on as per usual.
This may help:
https://addons.mozilla.org/en-US/firefox/addon/ramback/EDITED: 29 Aug 2017 14:22 by DSMITHHFX