Looks like it's probably this bug, which (to my reading) was a problem handling domain cookies:

https://bugzilla.mozilla.org/show_bug.cgi?id=950399

This would of course affect any site, but the two common instances were FB and google.

Here is the corresponding bug on Bugzilla : https://bugzilla.mozilla.org/show_bug.cgi?id=950399 This what related to the way Facebook and Youtube sends cookies for all their subdomains, and not just www. (see https://bugzilla.mozilla.org/show_bug.cgi?id=950399#c55).

The kind that affects a lot of users and is likely to be reported and fixed.