Say one's using GNU/Linux with a Tor daemon setup on a given local port (which happens not to be 9050). Web browsing traffic currently goes like this: vanilla Firefox -> Privoxy -> Tor.
Say one would like to replace Firefox with the Tor browser from the 'hardened' official TBB package. As the browser gets invoked via the launcher script, it starts its own bundled Tor proxy and it works alright. But telling it to use Privoxy or even existing Tor directly (via proxy settings) won't make it connect to the Internet.
How to configure the Tor browser to use already available Tor channels on the system?