2087194 - Use Windows accelerated app launches so links and Start menu launches reach an already-running Firefox faster

Status: ASSIGNED

(Toolkit :: Startup and Profile System, enhancement)

Description

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Windows 11 now lets a running app ask Windows to deliver its launches directly, instead of Windows starting a new process that finds the running instance, forwards the request and exits. Microsoft calls these accelerated app launches. They cover launches from the Start menu and taskbar, plus links for schemes the app is the default handler for. Developer docs: https://learn.microsoft.com/windows/apps/develop/launch/accelerated-launch

Edge is the first app using it, on Windows 11 25H2, 26H1 and 26H2 with the October 2026 non-security update (KB5127936) or later: https://blogs.windows.com/msedgedev/2026/10/29/edge-is-faster-on-windows-with-accelerated-app-launches/ Microsoft says most users see launches about 15% faster, and up to 33% faster when the machine is busy. They're also upstreaming the work to Chromium, so Chrome and the other Chromium browsers will presumably follow, and they're working on extending it to opening PDF files.

Firefox takes the slow path today. When Firefox is already running, every link click and every Start menu or taskbar launch starts a new firefox.exe (launcher process included) that hands the request to the running instance and exits. That's the normal case for anyone who keeps Firefox open, and launch on login makes it more common.

The API is a Limited Access Feature (com.microsoft.windows.acceleratedlaunch.1), but being unpackaged doesn't block us: the unlock code we already use for taskbar pinning unlocks it too. In a quick test with a local build, a link reached an already-running Firefox in about 3 ms instead of about 70 ms, and Start menu launches were delivered directly as well.

Open questions I can see:

• Which instance should take launches when several profiles are running.
• Whether Microsoft wants a heads-up before we use the feature, as our Limited Access Feature docs suggest.

Comment 1

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Attachment: WIP: Bug 2087194 - Use Windows accelerated app launches in the Windows remote server.

Lets Windows deliver Start menu and link launches straight to the running instance instead of starting a new process to forward them. Behind widget.windows.accelerated_launch.enabled, which is off by default and controlled by the windowsAcceleratedLaunch Nimbus feature.