... and Mozilla clones.
Open and use your Firefox browser. Then under "History" in the top menu, there is a link that says, "Clear recent History." Select "Everything" when you clear your history. You think that you have cleared everything, but have you?
Download and set up CCleaner from Piriform at
http://www.piriform.com/promo/ccleaner-100314?gclid=CKyC1M3mns0CFQ6LaQodQBECGQ - they have a free version. Then run CCleaner with settings to clear everything that you thought you just cleared off Firefox. You will see that Firefox has retained all kinds of info that you thought you had deleted. At least, that's how it works for me.
Does anyone know how to set Firefox so that it really deletes everything like the History says it does?
Just run it in a VM, make a spanshot with freshly installed and customized to your preferences Firefox (all addons, about:config settings, etc), and then roll back to that snapshot when needed (after each session, for example).