I know this might not fully be related to Linux, but I still think people here need to hear about this.
tl;dr uMatrix can leak cookies despite disabling them.
Install the latest dev build to mitigate.
Go here to test. It should say "No cookie found. Try reloading the page." under "Cookies sent to the server by your browser", if it doesn't, then that means you (inadvertently) have cookies enabled.