The Earth Browser plugin currently works only for IE and Mozilla browsers, including Firefox.
I'm using FF 2.0.0.14 and it works. On my 1.6ghz Centrino based laptop it takes about 18 seconds after clicking on the earth button, for the map to be displayed as a globe. It takes a LOT! longer if I have the laptop set for maximum battery saving mode.
Link:
Google Earth Browser Plugin
