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: 
Writing to a file is not possible, you'd have to write a server-side script and make a request to that script. Reading is possible if you use an iframe with the text file's location as source, and reading the iframe contents.
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!