On Unix, Firefox has had a long standing feature where you could remote control a running Firefox instance. This has traditionally worked through X properties (with two generations of protocols), which has the nice advantage that it works from remote machines as well as your local one, provided that you're forwarding X. Since I read my mail through exmh that's running on one of our servers, not my desktop, this is pretty useful for me; I can click on a link in mail in exmh, and it opens in my desktop Firefox. However, working through X properties also has the disadvantage that it naturally doesn't work at all on Wayland. Since Wayland is increasingly important, last November or so the Mozilla people fixed this by adding a new D-Bus based protocol (it landed in bug 1360560 and bug 1360566 but has evolved in various ways since then).

On current versions of Firefox, you will find this service on the session bus under the name org.mozilla.firefox.<something>, where the <something> is often 'ZGVmYXVsdA__'. In general this weird thing is the base64 encoded name of your Firefox profile with a few special characters turned into _, and that particular name is, well:

; echo -n default | base64
ZGVmYXVsdA==

Because this directly encodes the profile name in something that you have to get right, the D-Bus based version of Firefox remote control will reliably restrict itself to talking to a running Firefox that's using the same profile; the X properties based version doesn't always (or didn't always, at any rate). You can force a new Firefox to not try to talk to an existing Firefox by using --new-instance, as before.

(One case where you might need this is if you're testing an alternate version of Firefox by manually setting your $HOME to, eg, /tmp/ffox-test.)

It turns out that which protocol Firefox uses when is a bit tangled. If Firefox is built with D-Bus support, a running Firefox on X will be listening for incoming requests using both D-Bus and the X properties based protocol; you can talk to this Firefox with either. In the current Firefox code, if you built with both D-Bus and Wayland support, the client Firefox always uses D-Bus to try to talk to the running 'server' Firefox; it doesn't fall back to X properties if there's no D-Bus available. If you built Firefox without Wayland support, it always uses the X properties based protocol (even if you built with D-Bus, and so the running Firefox is listening there). You can see this sausage being made in StartRemoteClient() here.

This logic was introduced in the change for bug 1465371. Before then Firefox tried to use the X properties based remote control if it was running on X, and fell back to the D-Bus protocol otherwise. In thinking about it I've come to believe that the logic here is sound, because in a Wayland session you may have some programs that think they're running in X and then pass this view on to things run from them. D-Bus is more session type agnostic, although it only works on the local machine.

Note that this implies that you can no longer use Firefox itself as a client on a second machine, at least not if your second machine Firefox is a modern one that was built with Wayland support; it'll try to talk D-Bus and fail because your running Firefox isn't on that machine. If you want to remote control Firefox from a second machine, you now want a dedicated client like my ffox-remote program.

(Hopefully Mozilla will leave the X properties based protocol there for many years to come, so my cross-machine remote control will still keep working.)

Sidebar: some D-Bus protocol details

The D-Bus object path is /org/mozilla/firefox/Remote, which has one org.mozilla.firefox method, OpenURL(), all of which you can see by using a D-Bus browsing program such as d-feet. In the Firefox source code, what you want to look at is widget/xremoteclient/DBusRemoteClient.cpp (the client side, ie the firefox command you just ran that is going to pass your URL or whatever to the currently running one) and toolkit/components/remote/nsDBusRemoteService.cpp (the server side, ie the running Firefox).

Despite the fact that D-Feet will tell you that the argument to OpenURL() is a string, in actuality it's an entire command line encoded in the same annoying binary encoding that is used in the current X property based protocol, which you can read a concise description of in nsRemoteService.cpp. Presumably this minimizes code changes, although it's not the most natural D-Bus interface. This encoding does mean that you're going to need some moderately tangled code to remote-control Firefox over D-Bus; you can't fire up just any old D-Bus client program for it.

The client code for this is in toolkit/xre/nsAppRunner.cpp, in the StartRemoteClient() function.