This add-on works fairly straightforwardly. Once downloaded, if you select some text and right click on the text, you get a highlight option. The blurb states that if you use [Shift] and right-click you can choose your highlighting color, but I found that this did not always function reliably on my setup (Firefox 3 running under Vista). Worse, when I moved away from the page, the highlighting was lost, which seemed to undermine this add-on’s usefulness. A glance at the version date suggested that it had not been updated since 2006, so this may account for some of the limitations although it does validate as version 3 compatible.