These builds still have distribution-specific in-built API keys for some of the built-in services such as Google Safebrowsing, Google Location Services and Mozilla Location Services. See [1], [2], [3], [4], [5] and [6] for details and examples.
Additionally, upon first launch of Firefox, a unique client identifier is created, and this is sent to Mozilla by default probably before you get a chance to disable telemetry features within the preferences dialogs. See [2], [7], [8] and [9].
As these privacy impacting features are enabled by default, before first launch of Firefox on Linux, you should disable these third party and telemetry features and also lock down other security and privacy settings. See [10] and [11] for the method of doing so, and an example user.js that contains decent documentation on well over a 100+ recommended configuration changes to make Firefox more respectful of privacy and security. If you don't reconfigure a user.js before first launch, at least the "New Profile" event will be notified to Mozilla with the unique client ID after a delay of only 30 minutes from creation of the first Firefox profile[9] (first launch).
