I do not have FireBug. I know it is strongly recommended. I tried to download it the other day
but was having issues downloading it. Did you get it from https://getfirebug.com/
I turned off the Project Manager and invoked it from VS2010. Now it works fine.
Very strange... Its difficult to know what is the IE8, what is Sitefinity, etc. at the moment.
I will keep you posted.