Running different instances of Firefox side-by-side

Dave Townsend dtownsend at
Wed Mar 13 21:14:51 UTC 2019

A quick update here. After hearing some feedback from folks I've filed the
following bugs that I should have a patch up for in the next day: Don't show the profile
manager when the default profile was selected and an existing instance is
running. Return -new-instance
to its previous behaviour.

On Mon, Mar 11, 2019 at 11:35 AM Dave Townsend <dtownsend at>

> Woah this email got long. How Firefox considers whether to pass off to an
> existing instance of Firefox or continue launching a new one turns out to
> be more complex than you might expect. I'm mostly interested in making
> folks aware of and giving feedback on how this works after I've changed
> some things so feel free to jump down there. But I figured some folks might
> find some context in how things work currently. For that, read on!
> One of the goals of pushing to a profile-per-install model for Firefox is
> allowing users to run different versions of Firefox side-by-side without
> the additional hassle of editing shortcut files or running from the command
> line. This has meant changing the "remoting" code, which searches for
> existing instances of Firefox and passes command line arguments to them
> instead of starting up normally. I landed the changes to this a couple of
> days ago and I thought it was worthwhile explaining what has changed since
> it might not be exactly what you expect. And if that is the case figure out
> whether it makes sense to make any changes.
> *So first, a quick recap of what remoting has done in the past, because it
> varies from platform to platform...*
> OSX is the easy case. Firefox doesn't handle remoting at all. OSX does it
> all, assuming you are running Firefox by running an app bundle or a dock
> icon. OSX sees that an existing Firefox is running and just sends it a
> message, a new Firefox instance doesn't even start. I've made no changes
> here.
> Windows is the slightly more complex case. When run Firefox attempts to
> find an already running Firefox. If one exists it passes its command line
> off to it and quits. The -no-remote command line argument is a way to
> bypass this behaviour, running with it will stop the new Firefox from
> attempting to find an existing instance or becoming and instance that can
> be found by other instances. Basically there can only be one Firefox open
> that can be found by future invocations. The -new-instance command line
> argument is parsed on Windows ... and then ignored.
> Finally there is Linux. The more exciting case. Unless -no-remote or
> -new-instance are passed on startup linux will search for an existing
> version of Firefox based on a few criteria .. which varies a little
> depending on whether we're using dbus remoting or X remoting. We use X
> remoting if we are using X11 windows, and dbus if not (and dbus is
> supported). In both cases on startup Firefox attempts to find an existing
> instance of Firefox with the same remoting name (or you can provide a
> different remoting name with -a on the command line). dev-edition has one
> remoting name, all other versions of firefox have a different one. If there
> is more than one .. which one wins seems undefined. You can additionally
> pass "-P <profile name>" in which case Firefox will only select an existing
> instance running the named profile. On X remoting there are a few extras.
> Passing "-a any" on the command line will find any running Firefox
> regardless of remoting name. Passing "-u <username>" will consider
> Firefoxen run by the given user (otherwise it only looks at those run by
> the current user). -no-remote means FIrefox doesn't register itself to be
> found by future instances. -no-remote or -new-instance means we don't look
> for existing instances on startup.
> So that's all rather complicated. To make matters more fun the linux and
> windows implementations are handled by totally separate code running at
> different times during startup. The two key problems here were that windows
> completely didn't support more than one instance running, unless all but
> one were -no-remote, and linux was horribly complex and again unless you
> ran with command line arguments didn't support more than one Firefox at a
> time. We wanted something that allowed running Firefox release and Firefox
> beta and Firefox nightly with no special arguments at the same time.
> So I have done three things. Removed support for some of the things Linux
> supported. Made the code a lot more shared between windows and linux so
> things happen at the same time regardless of platform and both platform
> have what should be identical behaviours. Changed the order of when some
> things happen.
> What did I remove? Support for remoting to a different remoting name and a
> different user. Both seem unlikely to be useful for normal use cases, the
> latter frankly feels like a security risk.
> *How does it all work now?*
> OSX hasn't changed, maybe we'll want to do some changes here, but for now
> it already allows running different versions of Firefox so long as they are
> using different profiles, which is the default. So for the rest of this
> assume I'm talking about Linux (dbus or x11) and Windows. They all should
> behave the same.
> The new remoting does everything based on profile. When starting Firefox
> we do normal profile selection, which includes considering any -P and
> --profile command line arguments. Once we've selected a profile we attempt
> to find an existing Firefox instance using that profile. If one is found we
> send it our command line arguments and quit. If not continue start up.
> Since different installs of Firefox use different profiles by default this
> generally means that running Beta would pass off to an existing Beta. Same
> for other installs. It also means if you do "firefox -P foo -url
>" we'll open that url in profile Foo, either by using an
> existing Firefox using profile Foo or by starting with profile Foo.
> -no-remote and -new-instance still exist. Right now they do the same
> thing, they make Firefox not look for existing instances and not listen for
> remoting from future instances. They are pretty pointless now though, the
> only case where they would have an effect is when a second instance is
> trying to use a profile that is already used by an existing instance ... at
> which point we'll show the profile locked dialog on startup and refuse to
> startup anyway.
> The most visible side-effect that folks have started seeing from this
> change is caused by waiting for profile selection to occur before
> attempting to remote. If Firefox is configured to always show the profile
> manager on startup then attempts to open links from outside apps will cause
> the profile manager to show, because that is what selects the profile.
> Selecting the profile of an already running Firefox from the UI will then
> remote to that Firefox (barring a bug that should be fixed in the next
> nightly), but this is a change in behaviour and honestly not one I'd
> spotted before landing. In some ways the new behaviour kinda makes sense
> (if there wasn't already a Firefox running you'd get the profile UI
> previously too) but I can see how it is confusing too so it might be worth
> considering changing something here, we'd just have to figure out what
> profile we should use in this case.
> The other thing that might be confusing is that the version or install of
> Firefox you try to launch doesn't affect which version or install of
> Firefox you might end up remoting to. This has always been the case on
> Windows and normally the case on Linux, unless you pass an extra command
> line argument though so I'm not too concerned here.
> Hopefully this all makes sense. I'd like to hear if folks think that this
> is the wrong way to support this and if you spot any issues with it that I
> haven't.
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