I just completed my testing for Firefox 4 and PassiveRecon is ready to go as I just updated the version information with Mozilla. One point to note about Firefox 4 though is that they have removed the status bar. So the ability to launch PassiveRecon is now only limited to right clicking the page you are on and then selecting the PassiveRecon menu from there.
Let me know if you have any issues with FF4 and PassiveRecon, but I doubt you will.
Also, please send me any requests for updates/enhancements!