Using a system-installed Tor process with Tor Browser:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Tor ControlPort password should be given inside double quotes, inside
single quotes, i.e. if the ControlPort password is “secret” (without
curly quotes) then we must set the environment variable *exactly* like
this:

TOR_CONTROL_PASSWD='"secret"'

Yes, the variable MUST be double-quoted, then single-quoted, exactly as
shown. This is used by TorButtom to authenticate to Tor's ControlPort, and
is necessary for using TBB with a with a system-installed Tor.

Additionally, if using a system-installed Tor, the following about:config
options should be set (values in <> mean they are the value taken from your
torrc):

SETTING NAME VALUE
extensions.torbutton.banned_ports [...],<SocksPort>,<ControlPort>
extensions.torbutton.block_disk false
extensions.torbutton.custom.socks_host 127.0.0.1
extensions.torbutton.custom.socks_port <SocksPort>
extensions.torbutton.inserted_button true
extensions.torbutton.launch_warning false
extensions.torbutton.loglevel 2
extensions.torbutton.logmethod 0
extensions.torbutton.settings_method custom
extensions.torbutton.socks_port <SocksPort>
extensions.torbutton.use_privoxy false
extensions.torlauncher.control_port <ControlPort>
extensions.torlauncher.loglevel 2
extensions.torlauncher.logmethod 0
extensions.torlauncher.prompt_at_startup false
extensions.torlauncher.start_tor false

where the '[...]' in the banned_ports option means "leave anything that was
already in the preference alone, just append the things specified after it".