Question: I know the question has been asked thousands of times, but I'll ask it again: is there a way (even patchy) to write/read a dumb text file with Javascript or Protoype ?

This is only for debug purposes, and is not designed for production. The thing is I need it to work with (at least) both Firefox and IE (preferably under Windows).

Thanks in advance !

Answer: 
Javascript in browsers doesn't allow you to write local files, for **security reasons**. This **may change with time**, but as for now you have to **deal with it**.
What about cookies? It is accessible via javascript, it is on your client and it is a plain text file. Only issue is the size of it (4k max if I remember well).

What you can do as well is use your browser localStorage / userData / globalStorage (depending on your browser version). It acts like cookies (new webStorage / HTML5 specs) but can handle bigger amounts of data. Then, using some add ons (firebug on firefox for instance) you can easily read / copy / past the value and do whatever you have to do with it!