Logging in a release branch

I’m chasing a bug and am trying to debug the code. When starting with my debugging I build the default repository ( https://hg.mozilla.org/mozilla-central ). There I could enable logging by setting these environment variables:

export MOZ_LOG_FILE=/tmp/log1
export MOZ_LOG=signaling:4

After realizing that the behavior in that firefox version is a different one, I’m trying to debug the 68.x version.
I cloned https://hg.mozilla.org/releases/mozilla-esr68 and
after some time I found out that release versions do not support logging. So I changed the version in the config/milestone.txt file from 68.8.0 to 68.8.0a1. But I still don’t see any output in one of the /tmp/log* files. What did I miss?