make sure that your settings in preferences > privacy are set as it is described here: https://support.mozilla.org/en-US/kb/Blocking%20cookies#w_block-cookies-f...

Yes, I have cookies blocked by unchecking the box next to "Accept cookies from sites". However, cookies are still being set by almost every site I visit.

when you are viewing the details of those cookies through page info > security > view cookies - are they expiring at the end of the session or at a custom set date & time?

Some expire at the end of session others have an expiration date.

hmm, this is strange - maybe these cookies were already set before you disabled cookies for all sites. maybe try to clear all cookies and see if new ones are set for non-whitelisted pages afterwards...

Nope, I continue to delete cookies from each domain that I don't want. If I return to the site they are re-set in the browser. I cannot explain this and it is not the way the 3.6.X version I have been using behaves. I have avoided upgrading Firefox for this reason but keep hearing about security issues and thought I would attempt an upgrade again.

Okay, here's the weirdest part, when I visit a non-whitelisted site the site is automatically added to my white list!

okay, this would rather point to one of your addons going berserk. you might want to disable them one by one, restart the browser and inspect if the cookie setting behaviour has changed in order to find the culprit...

I disabled extensions one-by-one checking the behavior each time. The cookie issue continued until all of the extensions were removed. I re-enabled them one-by-one, checking each time, but now I cannot reproduce the issue even with all extensions re-enabled.

I cannot imagine what the cause/solution was. Thank you for your assistance with this.