Tor Browser: open URL from command line

I'd like to make Tor Browser my default browser by symlinking /etc/alternatives/x-www-browser to it.

However, using /path/to/tor-browser_en-US/start-tor-browser https://check.torproject.org/ results in the error message:

Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

Using /path/to/tor-browser_en-US/Browser/firefox gives the same result.

Trac:
Username: ilf