I'm not sure what you mean by that exactly. Precisely speaking, you can't "cancel" Firefox. It isn't a service that's purchased and able to be cancelled. I'm guessing that you want to know how to uninstall Firefox. If you want to do that, you can follow these instructions to [[Uninstall Firefox from your computer]]. If you're asking how to close Firefox after the application has stopped responding, the instructions to do that vary depending on the operating system (Windows, Mac, Linux, etc.) that you are using. On Windows, you can right-click on an empty area of the taskbar (usually at the bottom of the screen) and select the Task Manager. In the Task Manager, you can select Firefox and click on End Task to force it to quit.