If you open an application when it's already open, Windows creates a whole new instance of the application instead of just switching to the already-open application.
> If you open an application when it's already open, Windows creates a whole new instance of the application instead of just switching to the already-open application.
This can be handled by the application though. VLC for example has an option ("Allow only one instance") so that it will only let you, uh, open one instance at a time - if you try to open VLC again while there's a Window already open it just (IIRC) sets focus to the existing window.
I think Firefox opening a new window is an intentional design decision.
I don't think that's what's going on here. I'm pretty sure they're measuring the performance of the first window opened. A lot of Windows applications, Firefox included, do single-instance manually by opening an IPC connection to the old instance and telling it to open a new window. Also, DLLs in Windows are only loaded from disk into memory once. If XUL.dll is already in memory from another instance of the application, it won't be read from disk again. (It won't even get rebased for ASLR again.)
Kind of baffling to hear multiple application instances described as "weird," though. I guess that might be the case from a Mac perspective. Having a separate process per application window is fairly standard in Windows and desktop Linux. It gives you a simpler programming model, since per-window things can be global or singletons, and you can always terminate the process when the user presses the close button, rather than having to check for other windows. It also makes you more robust to application crashes, since only one window will crash.
And what if you don’t want the process to close? What if you want to open a new window after closing the others? On Windows you have to wait for the whole app to start up again.
That's another thing I'd argue is "weird" in macOS, the only widely used modern desktop operating system that does things this way, rather than vice-versa. Most new converts from Windows or desktop Linux to macOS tend to accidentally leave apps open because of this, since the only indication they're open is a tiny dot in the dock, and most of the time, the only remaining UI once everything is gone is a menubar. For an otherwise document-oriented UI, the fact that an application can be open with no documents is a strange concept that requires some getting used to.
