
When accessing the plug-in homepage using Shiretoko, Google tells you that your Browser version is not supported. Of course there is a reason for that. Messing around with untested software, especially with plug-ins, might hurt your user experience or data in ways I cannot predict here.
That said, the recent nightly builds on the Firefox 3.5 / Mozilla 1.9.1 branch seem to be pretty stable, at least on Mac OS 10.5. To get it running, you have to fire up about:config and change the key general.useragent.extra.firefox to something like Firefox/3.0.10. Now reload the Plug-in Homepage, download and install the Google Browser plug-in. After that, you have to keep the modified user-agent or the plug-in will cease to work.
Keep in mind that this might affect which add-on versions will be offered to you at addons.mozilla.org. Come to speak of it, I recently tried out the Minimap Sidebar that kind of gives your browser a Google Earth in the sidebar (or in a full tab). Really great: It allows you to switch between Google Maps, Google Earth and the awesome OpenStreetMap at any time without loosing position or bookmarks. It also integrates with various other location based services such as WikiMapia or loc.alize.us.