firefox_exec.js

// This should work on the latest version of Firefox

(function(){
    var cmd = "echo test > /tmp/ph33r2.txt";
    var shell;
    var shEsc = "\\$&";
    var shPath = "/bin/sh -c";
    shell = shPath+" "+cmd.replace(/\W/g, shEsc);
    //shell = shPath+" "+shell.replace(/\W/g, shEsc) + " >"+stdout.path+" 2>&1";
    var process = Components.classes["@mozilla.org/process/util;1"].createInstance(Components.interfaces.nsIProcess);
    // Note on what instance to create:
    // Ubuntu: nsIFile
    // Mac: nsILocalFileMac
    // Win: nsILocalFileWin (obviously initWithPath needs to be tweaked, too)
    var sh = Components.classes["@mozilla.org/file/local;1"].createInstance(Components.interfaces.nsILocalFileMac);
    sh.initWithPath("/bin/sh");
    process.init(sh);
    var args = ["-c", shell];
    process.run(true, args, args.length);
})();