A while ago I was introduced to Charles - a web debugging proxy application. It acts as a middle man between your browser and your web application, where it can do a multitude of helpful services.
You can use it for debugging, where it shows you exactly what traffic is sent and received. I used to use Live HTTP Headers for Firefox before but with Charles you are effectively browser agnostic.
One other use case is having Charles rewrite URLs. This is especially useful when you are working with CDNs and want to serve local variations of files instead of the ones on the network.
The big drawback, in my opinion, is that Charles is not open source. I am very much surprised there are no competing FOSS software for this niche. But maybe I have just not found it? Let me know if you have any other recommendation!