Tool to highlight and annotate text on websites?

I find a better one: Diigo

Try Wired-Marker:

It is a permanent (indelible) highlighter that you use on Web pages. The highlighter, which comes in various colors and styles, is a kind of electronic bookmark that serves as a guide when you revisit a Web page. The highlighted content is automatically recorded in a scrapbook and saved.

http://amb.vis.ne.jp/mozilla/scrapbook/

This extension also has the functionality you're after. It's more about saving pages than simply highlighting some text for mater on though.

There's quite a few sticky-note/highlighter add-ons. One of the nicer ones is Diigo. It has the option to integrate with the Diigo website so you can keep track of your important selections later on.