If you use Firefox in one Space and other apps in other Spaces, you might have noticed the following bug:

If you hit command+tab to switch from another app to Firefox, and this involves moving to a different Space, the Firefox window will not be selected (in focus), so you have to click somewhere in the window to use it, e.g. to type something.

+

Clearly, the purpose of a keyboard shortcut is to do something quickly without having to use the mouse. But command+tab app switching won’t let you do that with Firefox and Spaces. It’s not a problem when either the app is in the same Space (if there aren’t other windows of the same app open in other Spaces) or when Spaces is turned off.

This bug has been reported by other users. You can report it to Apple using their feedback form, and hopefully it’ll get fixed soon.

One workaround for now is to put Firefox in its own Space, then hit ctrl+# (# being the number of the Space Firefox is in) to switch back to Firefox. Cmd+tab will still get you to your other apps. Ultimately, this may be faster if you learn the behavior, because you’ll never have to hit cmd+tabtabtab to scroll through your open apps until you find Firefox. I’m experimenting with having 9 Spaces, with Firefox in the center Space (#5).