Using Linux FF38.0 (to which it just updated this morning ... perhaps I should play waiting game and see whether the issue persists now). I know my main laptop definitely has been having the issue, as has the netbox (both with this site, and this site only); not entirely certain about the heavy-duty desktops, notebook, other laptop or cell. Cell actually [i]has[/i] been set to not store cookies, so logout on closing browser is currently the expected behaviour. I could try Chrome on netbox and main laptop and see whether it still logs out every now and then, but I'd prefer not to spawn additional windows (Chrome doesn't support some addons I need on a semi-constant basis and breaks the outlay of some sites I use, so switching to it as a main is pretty much out of the question).
	Nothing in laptop/netbox browser settings (on Linux-side, anyway) that should cause such behaviour, haven't removed any site cookies manually,[i] et cetera, et cetera. [/i]

	(...I am definitely not going to purge my rather customised browser just to see whether reinstalling it would fix something it is extremely unlikely to fix and which only occurs on one site.)