When writing filters for the Firefox Add-On ‘Adblock Plus’ you can write rules to completely remove certain HTML elements from the page, but filtering criteria is in fact limited to a handful of things, like class and id names and attribute values.
What I was hoping for is say a Firefox Add-On which would pass the HTML for a page to some arbitrary process you specify, where this process could reconstitute the HTML for the entire page in any arbitrary way and then have the browser display that. Is there a Firefox add-on that allows this or is this sort of operation commonly accomplished by some entirely different but well-known means (and perhaps not browser-specific).
Wouldn’t this allow you to augment pages coming from some website to your browser with arbitrary new features, maybe from an entirely different website.
You are looking for Greasemonkey.