Hello - I've installed GRW on two different machines via the download at Mozilla Add-ons.
The plug-in appears in my status bar, and it occasionally changes colors, so I assume it is working. However, it never displays any notifications, so the only function it serves is to provide a one-click method for opening GR in a new tab.
I'm using Firefox 3.6.13 on Win XP here at work. At home, it's a Mac running 10.6.6 with whatever is the latest official Firefox releases (not 4 beta). I did not change any of the GRW preferences, apart from adding my Google log-in credentials.