I've been working on a maps toolbar for Firefox called 'MutantMaps'. The idea is to allow navigation between all of the popular mapping sites while preserving your longitude, latitude and zoom levels. At the moment Google Maps, MSN Virtual Earth, MultiMap, TerraServer and 192.com are supported. There is also support for navigating from Google Maps/MSN Virtual Earth 'mashup' sites (ie this kind of thing).
You can see from there screenshot below there is a drop down menu that allows navigation to a few selected Google Maps 'mashups'. It is a pretty random selection of sites that caught my eye and work well with the toolbar. If there are any other sites you would like to see added, please let me know.
You can install the Firefox toolbar from this page.
This is still early days for the toolbar, so please check back here for updates. Let me know how you get on and what else you would like to see.