In my case, enabling studies did nothing. According to the Mozilla people, it can take up to 6 hours for a study to be installed after you configure Firefox to allow them. I don't know if the browser needs to be running the entire time.If you can't wait for it to show up (it didn't happen for me even though about:config says I have Normandy enabled), you can get it straight away by going to Firefox Preferences > Privacy and Security > Allow Firefox to send technical and interaction data to Mozilla > Allow Firefox to install and run studies. Within a few seconds my extensions were back. This can be turned off again, if you like, once the extensions are back.
If you can't wait (I gave up after waiting most of the day), I found a discussion thread which includes a direct link to the XPI that Mozilla is pushing out with the Normandy/Studies system (hotfix-update-xpi-intermediate @mozilla.com.xpi). If you install it, you should get the same hotfix that Mozilla is pushing out through the Studies mechanism.
Once Mozilla pushes out a proper fix, you can use Mozilla's manual uninstallation procedure to delete the hotfix, if its continued presence bothers you.