I too use Debian's Firefox ESR. I noticed the "Allow Firefox to install and run studies" option in Privacy & Security Preferences a long time ago. It was unchecked and greyed out (i.e., unclickable), and a label below it says "Data reporting is disabled for this build configuration", so I gave it no further thought. This morning I woke up and launched Firefox, noticed this headline, and then noticed my extensions were still running. I looked in about:config and lo and behold, app.normandy.enabled=default [true]. I'll be filing a bug with debian to disable this in the build configuration.
Edit: There are some questions about whether Normandy is really enabled in Debian Firefox ESR even if the about:config setting defaults to true. I've filed a bug report, and I'm sure once a Debian maintainer has a chance to look at it we'll find out the answer.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=928433
Edit2: It should go without saying, but please do not spam this bug report with "me too" and its ilk.