title: chrome.webrequest.onbeforerequest.addlistener for Firefox?
body: <p>Google Chrome contains some functionality in its chrome.webrequest API (e.g., 
  <a href="http://developer.chrome.com/extensions/samples.html#12a7bf1490a26359eadf10917e37c5b9">http://developer.chrome.com/extensions/samples.html#12a7bf1490a26359eadf10917e37c5b9</a> )
  that can be used to redirect certain URLs to a specified web page. The Chrome extension uses a blocking event listener (chrome.webRequest.onBeforeRequest.addListener) and does a redirect for targeted URLs. How might I do something similar in a FireFox Add-on?</p>