Works with FF3 but... Rated 4 out of 5 stars

Yes you can get it to work with V3 by editing the "install.rdf" file to read "maxversion=3.0.1" instead of 1.4, but the toolbar icon is messed up as mentioned by others.
It actually is so messed up it has about 24 icons it wants to add to your toolbar, the entire icon set for one of the themes is added if you try this.

The entry under the File menu is there and works properly, so it still has that functionality at least. Too bad you can't get the option to change the icon for this as that may give the ability to get just 1 icon added to the toolbar even if it isn't the right one.