ObservePoint comes with a free browser plug-in that can be used as a sniffer in your Firefox browser.
Download the plug-in.
Once installed, an icon similar to Httpfox or Httpwatch will appear in the bottom right corner of the browser. When clicked, a side bar will appear revealing the OP browser menus: Session Recorder, and Page Requests.

Page Requests features the following tabs: HTML, CSS, JS, Images, Flash, and Tags. Each of this tabs, when active, each will show traffic sniffed from the respective category. The Session recorder is used to record session to allow pixel monitoring beyond password-protected pages (see below section for more details.)