When we did this and pointed the tool at a specific URL, the first thing we noticed was Firefox firing off a bunch of requests to various locations on the target site fishing around for the favicon, most of which (kind of by definition) ended with a 404. I'm surprised that for as long as I have used Firefox attached to a proxy for testing and debugging, I had never taken note of this before. In this case, it's really annoying -- after all, one of the points of using the proxy in the first place was to verify that all requests came back with a valid response.
The first thing I considered was adding special-case code for all favicon requests in the handlers for the proxy. That seemed kind of lame, so the second thing I thought about was looking for a way to suppress the requests in the first place. It turns out that there are Firefox preferences for this -- browser.chrome.favicons and browser.chrome.site_icons -- as described here.
I have two main points left for this post. First, I wanted to show the code for how set these preferences in Selenium WebDriver when you build your FirefoxProfile:
Second, I will pitch my opinion that in most automated testing scenarios, you probably ought to be suppressing favicon requests from Firefox, even if your site actually has one. A good way to do this is to save the preferences into a custom Firefox profile used for automated testing. If you don't already have one of those, you should. I will try to provide some tips on how to maintain and use one of those in a future post.