Testhammer is a python package containing wsgi middleware based on Webob that I'm using periodically to gather request/response information and other details for websites.
I like firebug, but I am in the unfortunate situation of having to support IE6,7,8. Also, firebug(as far as I know) does not have the ability to save off the data for a session so that analysis can be done over time.
Imagine this situation. You have a website out in production that has been identified as being slow in some areas. You've been asked to investigate possible reasons for why. Add to this the fact that you have 60 other developers currently adding features that will potentially make it even slower.
So, you gather some data and identify a potential fix, develop a patch. How do you know whether it's faster or not? How do you know whether features being developed are going to make it slower or not.
My idea was to setup a proxy between my browser and the site being tested and capture all request,response traffic and save it off to a database(the http messages in their entirety) along with various other metrics I could capture over time as needed. Then I could run reports that would answer the questions with some certainty.