In Firefox 3.6, if the network.http.pipelining option is enabled (in about:config), then various Jenkins pages fail to load properly. The project configuration page is often affected - the symptom is that the "LOADING" overlay never goes away and you get a bunch of Javascript syntax errors in the Firefox error console. Looking at the Javascript files linked in the error console, it appears that the files are being received in a truncated form.

It seems like there is an issue with the built-in web server where it doesn't handle HTTP pipelining properly. The current Firefox versions do not use this by default, but future versions apparently will.