So the idea is that you dont really install firebug as a browser plugin but as a javascript library. Its easy to use and works really well. All you have to do is add a link to favourites like the one below:

javascript:var%20firebug=document.createElement('script');firebug.setAttribute('src','http://getfirebug.com/releases/lite/1.2/firebug-lite-compressed.js');document.body.appendChild(firebug);(function(){if(window.firebug.version){firebug.init();}else{setTimeout(arguments.callee);}})();void(firebug);

It to do it create a link in favourites, right click on it and edit properties pasting the url above. Other option is add it as favourite clicking on this link: Firebug bookmarklet

Once you have it in favourites go to the page you want to debug and click bookmark which will load firebug.

The only problem i noticed is that mouse wheel can sometimes make firebug not showing up so if you cant see it try to scroll up or down or find the firebug icon showing on the page.

Absolutely brilliant tool!! more information on the firebug website