Need a minimal, easily-modifiable HTTP proxy

[__BMR__]

I'm looking for a simple HTTP proxy that I can modify to fix the
"brokenness" of a website that I use often. One aspect of the
"brokenness" is that if I visit the site from two different tabs in my
browser, what I do in one tab will interfere with what happens in the
other. There are a few other minor issues I want to tweak to my liking.
I thought a simple local proxy would give me maximum flexibility when it
comes to the tweaking part.

http://www.xhaus.com/alan/python/proxies.html contains a list of
potential candidates but it's difficult to get a feel of the ease of
modification.

I'd like to hear your experience/recommendation if you have solved a
similar problem before.

Thanks.