It doesn't look possible right now. I've looked through the extension docs (http://developer.apple.com/safari/library/documentation/Tool...), and the overall architecture is very similar to Chrome's. Safari extensions are essentially user scripts with a limited API for access to the UI and user data; if Apple doesn't provide an API hook for a given UI feature, the extension can't touch it. SafariTabs would be possible to implement if Safari fired events when tabs and windows are closed, but it currently doesn't.