It’s a nice feature, and it does a lot to mitigate the lack of navigational functionality in the Gmail interface. Unfortunately, it requires using the current Gmail interface, which is not comparible with the CustomizeGoogle Firefox extension.
One thing I’ve never understood is that Gmail search engine doesn’t offer spelling correction. That seems like an odd omission from the company that popularized “did you mean?”. Does anyone know why this feature is unavailable in an application where it would be extremely useful?