For firefox 3 beta versions,firebug 1.0 is not compatibile.so try to install its beta version firebug 1.1.you can get it from here.
Among a lot of bug fixes, some great features of this version are:
* Supports Firefox 3,
* eval() debugging,
* browser-generated event handler debugging,
* executable lines marked with green line numbers,
* user-controlled naming of eval() buffers,
* clearer debugging icons,
* internal firebug debug output,
* external editor interface,
* Stack side panel on “Script” panel for callstack,
* CSS errors report against source lines
Please download and install Firebug 1.1 here.