It is apparently possible to use Firefox containers to bypass or enable a VPN on a per-site basis. I discovered this yesterday and it makes using a VPN nowadays much easier, wish I'd heard of it ages ago. Using a SOCKS proxy this way also reduces captchas.
To setup:
- Install Firefox Multi-Account Containers.
- Install Container Proxy.
- Add VPN config under Extensions (puzzle icon in toolbar) > Container proxy > Proxies (for example, Mullvad SOCKS5 proxy).
- Make sure "Proxy DNS requests" is checked and the Uncloak canonical names setting in uBlock Origin is disabled to prevent DNS leaks.
- Assign VPN to default and private browsing container.
- Create a new container named e.g. "Unsecured" with the Multi-Account Container add-on and assign it a direct connection (default).
- In VPN client, configure Firefox to use split tunneling. Example split tunneling with the Mullvad app.
- Test default and unsecured containers against VPN website.
The Container Proxy add-on is only needed to configure the unnamed default and private browsing containers. If you want to do the inverse (create a VPN container and leave the default unprotected), you can do that solely with Multi-Account Containers under Extensions (puzzle icon in toolbar) > Multi-Account Containers > Manage Containers > Container > Advanced proxy settings.