I've had this problem myself when booting/logging in for the first time. I click on the icon, and I get that message. It only happens occasionally for me. You'll need to kill Firefox in order to restart it. It's not that hard. Open the Xfce Terminal (or some other terminal emulator, such as Konsole) and enter "killall firefox." This should stop any processes named "firefox." According to the killall man (manual) page: "killall - kill processes by name," and "killall sends a signal to all processes running any of the specified commands. If no signal name is specified, SIGTERM is sent." If that doesn't work, you can run "htop" which is a table of processes. In htop you can see the processes running on your computer. Firefox might be running two or three. You should be able to select each one with the keyboard (or mouse) and use F9 to kill it. Once there are no firefox processes running, you should be able to start Firefox.
I hope this helps.