Shinobi-san on 15 September 2013
Having a look at the Firefox add-on SDK...not bad.
You can do something similar to this using the Page-mod module.
I would imagine you would pass through a list or array of users that you wish to ignore to your js/script file. Then using javascript search through the html for all divs with class post-body, that belong to those users, and delete it or change the contents of it to the message you have.
I need to play around with this shit! :)
