Buyer Beware: Firebug 1.6 + Firefox 4 Beta
With JQuery, this manifests through $.ajax calls: success and error handlers don’t fire.
The workaround is to uncheck the "Show XmlHttpRequests" option in the Console panel.
For more details, see Firebug's Known Issues page.
Hopefully this short post saves someone more time than I lost dealing with this bug today.