The motivation for looking for this kind of solution is mainly to explore ways of creating essentially a "browser game" that does not need to necessarily be natively programmed for each browser (write something that processes URLs and returns the page with appropriate game content rather than writing a toolbar for every web browser).
The project I'm currently working on is, from what I understand, is a copy of PMOG (Nethernet?) where players have these game items they can leave on the page. Currently, the programmer working on the project wrote a Firefox plugin that allows "leaving things on pages". The items left on pages are stored in a database and queries are done via RESTful calls.
The idea of looking for someway to intercept the requests is to make the game "browser agnostic" so I'm researching what ways are possible.
To be clear, this project is for a University course. I am actually interested by the reaction to my post. I was not aware that some technologies that can do the necessary functions could be considered dangerous. The actual tech that my professor suggested was not some cross site scripting malware, but a proxy.