Macos – How to open a new window from dock if it’s already running in OSX

You need to use AppleScript for this. The ideal solution would be to use a built-in function from Firefox, but it doesn't offer one – its AppleScript dictionary is very limited. So we have to emulate keyboard shortcuts.

Open up your ~/.bash_profile and add the following shell function:

function firefox-window() {
/usr/bin/env osascript <<-EOF
tell application "System Events"
    if (name of processes) contains "Firefox" then
        tell application "Firefox" to activate
        keystroke "n" using command down
    else
        tell application "Firefox" to activate
    end if
end tell
EOF
}

This will call osascript, which executes AppleScript commands, then activate Firefox, and then emulate a ⌘N keypress – but only if it's already running. If not, Firefox will just be opened, so you don't get two new windows. Also, you can exchange "n" to "t" to get new tabs, obviously.

Save the ~/.bash_profile file and enter source ~/.bash_profile to reload it. Then, just call the following function whenever you need a new Firefox window:

firefox-window

Of course, feel free to change the function's name.

If you want to be able to pass an URL argument from the command line, see this answer: How to open a new Firefox window with URL argument.

_{~/.bash_profile is where all your custom functions should reside. If the file doesn't exist, you can just create it.}

_{Shell functions are more powerful than aliases, as for example they allow you to use arguments too. You could theoretically pass the URL of the new window too, and then tell Firefox to open it with the OpenURL or Get URL command – but I haven't tried them.}

_{Regarding the syntax used: The <<-EOF is a here document, making it easier to pass multi-line input to osascript. The input will be parsed until the EOF marker appears again.}

If you right click the Dock icon & select Options > None
then later, right-clicking the Dock icon & selecting New Window will open on the current Space.

I'm not certain how well that will work if you have "Displays have separate Spaces" enabled (& I'm not messing up my 6-Space, 2-Display layout to find out;) in System Prefs > Mission Control.