Make sure that all Tracking Protection related features are disabled in Tor Browser
Mozilla's Tracking Protection landed in Firefox 35 (see: https://kontaxis.github.io/trackingprotectionfirefox/ and http://ieee-security.org/TC/SPW2015/W2SP/papers/W2SP_2015_submission_32.pdf for how it works).
privacy.trackingprotection.enabled is still set to
false in ESR 38 (and AFAICT there are no plans to having this enabled by default at all). We should make sure that all the other bits of that feature (like updating the blocklist every 45 minutes) are disabled as well.