When the tor-launcher extension starts Tor Browser's tor, it uses the "-f Data/Tor/torrc" argument and the "--defaults-torrc Data/Tor/torrc-defaults" argument (if a torrc-defaults file is found). Here's what the man page says about these options.
Specify a new configuration file to contain further Tor configuration options. (Default: @CONFDIR@/torrc, or $HOME/.torrc if that file is not found)
Specify a file in which to find default values for Tor options. The contents of this file are overridden by those in the regular configuration file, and by those on the command line. (Default: @CONFDIR@/torrc-defaults.)
So it looks like Tor should be copying the contents of torrc into torrc-defaults. Why not use one torrc file only and set it with either the -f or --defaults-torrc options? Why are both files included?