Bulkhead

Firefox containers keep your logins apart. Bulkhead keeps their connections apart too: each container gets its own SOCKS5 proxy. If that proxy stops working, the container's tabs stop loading instead of quietly falling back to your normal connection.

That last part is the whole point. A work tab, a personal tab and a shopping tab can leave your computer through three different servers at once, and none of them will switch back to your own address without telling you.

Why a proxy needs a kill switch

Firefox can be told which proxy to use, but it can never be told to use nothing at all. So when the proxy you picked goes down, Firefox quietly sends that traffic over your ordinary connection instead, and nothing on screen tells you it happened. Bulkhead watches for that and stops the container instead. You get a page explaining what went wrong, rather than a page that loaded from an address you didn't want it to.

Checks that actually check

Every few minutes Bulkhead confirms your traffic is really coming out at the server you picked, not just that something answered. If it comes out somewhere else, or nothing answers, or a check has not finished yet, that container stays blocked until it is proven working again. Nothing is remembered between restarts either. A freshly started Firefox trusts no server until it has checked it.

What you get

• A popup showing where the current container is going out, whether that has been verified, and the exit IP it was verified at
• A server picker with search, favourites and recent picks
• Your own SOCKS5 servers as well, such as an SSH tunnel or any proxy you run, held to exactly the same checks
• Tabs outside any container, and private windows, can each be pinned too
• A warning when a server you rely on goes out of service, with a one-click move to another one nearby
• Optional extra protection: WebRTC forced through the proxy, and Firefox's habit of connecting to links before you click them switched off. Both are reversible and put back exactly as they were
• No account, no analytics, no data collection of any kind

Mozilla's Multi-Account Containers decides which sites open in which container. Bulkhead decides how each container reaches the internet. Run both and a site can be pinned to a container, and that container pinned to a server, without either extension knowing about the other.

If your VPN provider hands out SOCKS5 proxies, they will work here. Mullvad's are supported directly, with their whole server list built in and kept up to date.

What you need

For Mullvad's servers you need a Mullvad account and their app connected, because those servers only exist inside the tunnel. Custom SOCKS5 servers need neither. There are also two Firefox settings worth changing by hand to close gaps no extension can reach. The settings page explains both in plain terms and gives you the exact values.

Free and open source, GPL-3.0. There is no build step, so the code on GitHub is character for character the code that runs here.

Unofficial. Not affiliated with, endorsed by, or supported by Mullvad AB.