nsIWebBrowser
the chrome may optionally implement nsiinterfacerequestor, nsiwebbrowserchromefocus, nsicontextmenulistener and nsitooltiplistener to receive additional notifications from the browser object.
... see also nsibasewindow nsicontextmenulistener nsidomwindow nsiembeddingsitewindow nsiinterfacerequestor nsishistorylistener nsitooltiplistener nsiuricontentlistener nsiweakreference nsiwebbrowserchrome nsiwebbrowserchromefocus nsiwebprogresslistener ...

Embedding Tips
make your chrome object implement the nsitooltiplistener interface.