Funsize is very close to be enabled in production! It has undergone a Rapid Risk Assessment procedure, which found a couple of potential issues. Most of them are either resolved or waiting for deployment.

To make sure everything works as expected and to catch some last-minute bugs, I added new update channels for Firefox Nightly and Developer Edition. If you are brave enough, you can use the following instructions and change your update channels to either nightly-funsize (for Firefox Nightly) or aurora-funsize (for Firefox Developer Edition).

TL;DR instruction look like this:

• Enable update logging. Set app.update.log to true in about:config. This step is optional, but it may help with debugging possible issues.
• Shut down all running instances of Firefox.
• Edit defaults/pref/channel-prefs.js in your Firefox installation directory and change the line containing app.update.channel with:

pref("app.update.channel", "aurora-funsize"); // Firefox Developer Edition

or

pref("app.update.channel", "nightly-funsize"); // Firefox Nightly

• Continue using you Firefox

You can check your channel in the About dialog and it should contain the channel name: