I'm searching for an utility (better free) or an add-on for Firefox that intercepts the http traffic and replaces or deletes some keywords in the html source, basing on a pre-defined user edited list. So that I could have a 'cleaner' web experience.
Example 1: I would like to replace every occurrence of "http://www.mybestwebsite.com/ugly-ads" with an empty string, so that I can still see the website "http://www.mybestwebsite.com", but I don't see anymore the ugly ads from that same website.
Example 2: I would like to replace every occurence of " nastyword " with "-censored-".
Does a utility like the one described, or a firewall with this kind of content filter ability, or an Add-on for Firefox with the same features exist?
EDIT: sorry, I forgot to mention: It should work under windowsXP and/or w7