After looking at Microsoft Activities in IE8 and noticing that they look suspiciously like my ideas around actions in Operator, I decided to implement them in Firefox.
For your downloading pleasure, here is a first pass at Activities support in Firefox. I haven't actually looked at IE 8, so what I did was based on the page about Activities I saw this morning. You can install activities from the Service Gallery, and then if you open a new window, those activities will be active (they don't become active in the page you download them from - working on that). To use an activity, highlight some text and right click on it.
If folks think this is interesting, I'll keep working on it. Enjoy.
Note this doesn't support previews yet - I'm working on that.