With the release of Firefox 39 today also comes the final release of the Firefox 31 ESR (barring any security updates in the next six weeks).
That means you have six weeks to manage your switch over to the Firefox 38 ESR.

If you've been wondering if you should use the ESR instead of keeping up with current Firefox releases, now might be a good time to switch. That's because there are a couple features coming in the Firefox mainline that might affect you. These include the removal of the distribution/bundles directory as well as the requirement for all add-ons to be signed by Mozilla.

It's much easier going from Firefox 38 to the Firefox 38 ESR then going from Firefox 39 to the Firefox 38 ESR.

If you want to continue on the Firefox mainline, you can use the CCK2 to bring back some of the distribution/bundles functionality, but I won't be able to do anything about the signing requirement.

https://mike.kaply.com/2015/06/30/the-end-of-firefox-31-esr/