Bulkhead от S4NT0S

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

Works with what you already use

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.