I decided to upgrade Fifefox browser to version 3.5.5 and also update all my add-ons.
Unfortunately I found out that FireBug add-on could not be installed correctly, it was asking me to restart the browser, no matter that I did it for several times!
I also uninstalled Firebug and also re-installed it, but no way...
On the web I found out that it was my anti-virus (in my case Kaspersky...), that did not allowed Firebug to be installed properly.
I suspended the antivirus for a few minutes and, voilà, Firebug add-on is back.