Join Date: May 2004
Location: near Bremen, Germany
I read that original post when you made it and… hm.
I don't think macOS has a mechanism for per-process routing.
You could use something like Docker, but on macOS, that's a VM anyway, and you wouldn't really have a GUI, so probably not practical.
There's browser with built-in VPN, such as Opera, but I'm guessing they wouldn't let you pass a custom config.
Your best bet… maybe do in fact run Firefox in a macOS VM?