After you have installed the AlertStopper extension in your Firefox browser, try visiting a webpage that causes multiple annoying alert dialogs. For the first alert, prompt or conform, you would see a normal dialog. But for subsequent dialogs, you would see an option to prevent that page from creating further dialogs. Using this option, you can tell Firefox to disable alert(), confirm() and prompt() functions for that webpage.
AlertStopper extension does not have any options. It just keeps monitoring all the webpages whether they are loading more than one of these dialogs. If you want to turn off the blocking of annoying dialogs, then you will have to type about:addons in Firefox address bar and disable the extension from there.
You can get the AlertStopper extension for Firefox from https://addons.mozilla.org/en-US/firefox/addon/alertstopper/.