if i understood you, you want that, when you go to a page, you're redirected to another one taking the link from the page in which you are.
Originally Posted by Torment
and you want to do it as a FF extension.
so i suggest you to:
1-read this tutorial about XUL, the language used by FF extensions.
it's quite easy to create an extension for FF, but only if you have any knowledge about DOM and JS...