I think about:config is enabled on Nightly (possibly Beta as well?), and also the Fennec F-Droid build.

It doesn’t launch the new binary at all. When the current process wants to create a new process, instead of doing fork+exec (which launches the new binary and wreaks havoc because versions now don’t match), it simply tells the ForkServer (a different process running the old binary) to fork (split) itself.

Chromium also does this; they call their equivalent to ForkServer the zygote process and this article explains it really well.