René-Luc just released an add-on for Firefox and XulRunner: MozMapLayers. It's based on an Open Source JavaScript Library : OpenLayers.
OpenLayers makes it easy to put a dynamic map in any web page. It now works in a XUL environment and provides new features as OpenLayers.Layer.mozStorage. Such a layer is read from a local SQLite database. The geometry must be stored as Well Known Text (WKT).
MozMapLayers brings easy ways to put dynamic map in XUL applications. You can try MozMapLayers via a little demo named liziLayers, to create and test OpenLayers scripts in HTML or XUL context.