Net connection can be safely excluded - not only is mine very good, but net connection problems aren't exactly prone to randomly eating pre-existing cookies. I'd otherwise expect safety settings, but I have triple- and quadruple-checked those from all angles now. It keeps cookies until they expire - and given that I've discovered myself still logged on on sites I've not visited in months or even over a year, it does just that. 
	Laptop is 64-bit Ubuntu(14.04)/Win 8 dual boot. Netbox is SUSE.  

	[@Melkor]: That's on to do list for certain. 

	EDIT: Well, I can now confirm that problem persists at least on laptop. Closed browser == no login session.