Just came across this issue. It appears that Firebug v2.0 is causing conflicts with Google Maps (v3). Notable it prevents layers

ReferenceError: rd is not defined

Turns out this is a known issue and has been reported to the Firebug team. (June 11, 2014 at 2:58:19 AM). Issue #7501

It only seems to affect Google Map Layers. The actual map itself will load and correctly resolve a given LatLng object.

It just appears to be a conflict between how Google Map injects the map "layers" (markers, icons, vector, data-layers etc...) and Firebug.