Running different instances of Firefox side-by-side

Marco Castelluccio mcastelluccio at
Thu Mar 14 13:19:20 UTC 2019

What is the bug where you made the initial changes?
We should link to the bug the regressions caused by it (I've seen at least a
couple regressions filed mentioning this post on dev-platform rather than
the bug where the regression was introduced).

- Marco.

Il 13/03/19 22:14, Dave Townsend ha scritto:
> 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>
> wrote:
>> 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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