This happends:
1. Firebug is injected in the page,which is mean,if the z-index of firebug is lower then one element of your page you wont see it again.
2. Firebug wont injected in your page util the page finished loaded.
The web inspector is better then firebug,it is hosted in MIHTool and do more then firebug,please try it out :)