Firefox – Change Firefox update channel from ESR to release

They chose to remove it http://www.browserscene.com/2011/06/firefox-dropping-its-channel-switcher.html

Converting to ESR is done in principle by updating in defaults\pref\channel-prefs.js this line :

pref("app.update.channel", "esr");

According to the mozillaZine thread Changing Your Update Channel, it's hard coded in the channel-prefs.js file, so changing in about:config will not work.

However, not every non-ESR version can be converted to ESR just by changing channel-prefs.js. The ESR version is entirely distinct from the normal version, even if the differences are very minimal for a given version, so one should use the released ESR version. Even if updating channel-prefs.js works for the some version such as Firefox 10.0, there is no guarantee that this will work for the next ESR version.

This is clearly shown in the article Enterprise/Firefox/ExtendedSupport :

You can see from the above image that not every Firefox version has a corresponding ESR version. There are much fewer ESR versions than "normal" ones. The article also says that testing if a new ESR version fits the needs of your organization is your responsibility.

Because of all these differences, I would advice against converting an existing Firefox installation to ESR, but would rather start from a clean ESR installation.