Web Marker Firefox extension is used in a collaborative effort. It allows you to mark (highlight) a web page or certain sections of web pages and get a link to those pages so you can share your highlights with other users.
This Firefox extension is good for having a copy editor mark up a web page and send the link back to the writer, telling them to fix their changes. "The highlighted part in blue needs re-worded. Send it over to me again when your done."
In order for both users to view the changes, both users should have the Web Marker Firefox extension installed.