I’ve made version 0.6 of Firefox Activities available in the Add-ons Sandbox. If you like it, please review it so I can get it moved out of the Sandbox.
This version has a working preview for Firefox 3, as well as management of activities. It’s very close to the Microsoft version. The only thing it is lacking at this point is the button that appears after you select text. I’m looking at that.
I also had to make Operator 0.9.1 available due to some translation issues and some interaction issues with Firebug.