Last beta of Mozilla's Add-on SDK
The final beta of the Add-on SDK has added a new Hotkeys API to allow for the creation of browser-wide keyboard shortcuts, a new WidgetView object which enables window-specific widgets and new APIs for content scripts. As Mozilla moves towards a model where add-ons are run in their own process, content scripts run in the same process as the web page and are used to communicate, over the new APIs, to the add-on's script running in a different process; this reduces the communication down to a more manageable process of emitting events and acting on the reception of events.
Other changes are detailed in the release notes. The Add-on SDK requires Python 2.5 or 2.6; Python 3.x is not currently supported. The SDK can be downloaded(direct download) from the Mozilla Labs FTP server and documentation is available online.