Google updated its favicon recently and of course it is coming to Firefox in a future release. But if you don’t want to wait  for the fix, here’s a tip to keep up with the times.

1. Download this updated version of the Google search plugin. (Only change is the updated data: url for the favicon)
2. Copy it to your searchplugins folder in your Firefox install folder  (typically C:\Program Files\Mozilla Firefox\ on Windows), and overwrite the existing google.xml file.
3. Restart Firefox.

Enjoy!