Did you try setting a system-wide proxy server under System Settings » Network » (select a connection) » Advanced… » Proxies?

I haven't tried that and that likely will work but I don't want to use that proxy system-wide. I want it specifically for this browser only.

	▲	johnisgood 4 days ago \| parent [-]
		If you want Tor, "torsocks" probably works. I am not sure if it conforms to or cares about environment variables such as "http_proxy" and "https_proxy", or through CLI: "--proxy-server", as some programs do.