I am writing a C program, which needs to start several firefox windows. The problem is: I need to kill them later, so I need the process id. So I started each of them (via fork and exec) with the argument "-new-instance", but get the error "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system." when a firefox process was already running (because there is just one allowed, I guess). Is there a way to fix this problem, or at least get the pid of the process firefox connects to if i don't start it with "-new-instance"? By the way: It just has to work on one computer, so I can adjust settings etc. System: Arch Linux (Manjaro) Thanks for your help