It's a shortcut to an Edit menu item within the application, so whatever shortcut the app gives it; e.g. in Firefox it appears to be simply Cmd-<space>.
But wait... that doesn't actually work (it just switches to the next input layout). Maybe that depends on my keyboard settings in System Preferences. Or maybe it's just a Firefox bug.
Not for me. (I suspect the details of this vary between system versions, and they certainly depend on settings chosen in System Preferences / Keyboard / Shortcuts.)
It's the standard behavior. If you do something to modify or override it of course it won't work, but that's like changing the keyboard to Dvorak and saying that pressing the g key gives an i instead so you can't count on getting a g when you press the g key.
Works fine for me in latest Firefox. I've never had it not work in a specific app. Though, sometimes the keyboard crashes and doesn't come back up until a restart (that could be the fault of my Ryzen Hackintosh, though).
