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).
Re:Firefox 36.0 released
by tachyonbeam on Tuesday February 24, 2015 @01:53PM
(#9102686)
The kind that affects a lot of users and is likely to be reported and fixed.
What sort of bug is this that only affects Facebook & Google?