I'm running Windows XP Professional SP3.
I enabled FirePHP and I went to
http://www.firephp.org/ and the Firebug icon bottom right was grey and I couldn't open the Firebug window, it wouldn't appear either within the Firefox window or a new window.
I disabled FirePHP and restarted Firefox and then the Firebug icon was in colour again and I could open it's window. I enabled the console and net panels and then re-enabled FirePHP. After restarting Firefox again the icon was again grey and again I couldn't open it's window.
However I then went to the back end of a Joomla site and by previewing a content item I got a popup preview window which is another Firfox window but without the menus and toolbars, only a non editable address bar.
Within this pop up window I could open the Firebug window and there I could see the little blue FirePHP icon next to the Firebug icon top left of window and by clicking on that I got the menu and clicked on "FirePHP Headquarters" which opened
http://www.firephp.org/ in a new tab in the pop up window and I got the logging messages.
Which led me to think maybe it's a compatibility issue with a toolbar, so after disabling and re-enabling my tool bars I've tracked it down to Yahoo Toolbar 1.6.2.20080910 which I don't even use but I think was installed as an "added extra" by Google Chrome.
So problem solved - I've no need of the yahoo toolbar and am happy to live without it.
Thanks for your help and I look forward to many happy hours debugging my code that much easier with your addon.
Cheers,
Alan.