OK, how would you analyze the packets being sent to you?
I think if you have to ask that question it will be way above you head.
First you have to sort through 1000's of packets to identify which ones are for the right session. Then put them in order. and Decode the packets.
Unless you know exactly what application the packet is for it is next to impossible.
I still don't get what you are ultimately trying to achieve.
If your script is to be running on the client's computer, then you don't need to do anything, as everything displayed on the browser is already downloaded onto your computer.
For a packet anaylizer try
BUT DON'T ASK ANYONE IN THESE FORUMS HOW TO USE IT, OR READ THE OUTPUT.
YOUR ON YOUR OWN