How to configure Firefox to only connect through VPN?

Question

VPN

System: Win7 64bit
VPN: SoftEther VPN Client + VPN Gate Client Plugin
or via OpenVPN
It is necessary that FF does not bypass the VPN. That is, when the VPN connection is broken, FF should not break directly.

Answer 1

Specify local dns address
127.0.0.1

Answer 2

Some VPN services have a similar feature that restricts the Internet in case of disconnection with the VPN server. The feature is called "Disable Default Gateway" or synonyms.
There is also an ancient utility (released during Windows XP and working mainly only on this OS) by a third-party developer for OpenVPN macrodmin.ru/2010/11/ip-filter-pod-windows

Answer 3

I love crutches, so:
1. Run the virtual machine.
2. We let all traffic through your VPN.
3. Set up a proxy server on the virtual machine.
4. Set up Firefox on your machine through a proxy server.
5. PROFIT!
UPD: By the way, you can write a simple script that will check the status of the VPN and, in the event of a break, shut off Firefox's oxygen.