I have a Firefox account and use it to sync across devices; being able to sync and then completely log out on one device is a primary reason I use Firefox. I log out of d…
I have a Firefox account and use it to sync across devices; being able to sync and then completely log out on one device is a primary reason I use Firefox.
I log out of devices that I do not own when I'm exiting Firefox. Today I opened Firefox on a device that I'm 100% certain I logged out of yesterday - and found that my bookmarks, bookmarks bar, and add-ons were already visible and active, and one of my add-ons was already logged in.
I verified that the Firefox Sync / login screen showed that no one was logged into the Firefox account, and then I logged in using my usual credentials. It reflected that I was then logged in, but everything else remained visible.
I followed Firefox instructions for viewing the logs, and strangely, there was no log. I went up a directory, and found that all the logs were in the 'failed' directory. Clicking on each of the failed subfolders shows only: ids 
This is very concerning because it entirely negates the security and privacy aspects of having a login. I was not logged into Firefox - and the Firefox interface confirmed that this morning, in addition to confirming it when I logged out yesterday - yet somehow not only were my personal bookmarks showing, but one of my add-ons was also logged in, providing further access to anyone who might have opened a Firefox window from this device.