Feb 14 2008
Ah, what a day! First of all I was re-installing Firefox after the rebuilding of my system lately and what do I see? Yes, I could no longer install Firebug or any other add-on available.
Always the same error occured:
Firefox could not install this item because "install.rdf" (provided by the item) is not well-formed or does not exist. Please contact the author about this problem.
This was always followed by:
Firefox could not install the file at >file location< because: Unexpected installation error. Review the Error Console log for more information -203
Well, my first idea was that the installation went wrong, so de- and re-installing Firefox, but the problem was again there. Afterwards I have tested to install Enigmail and some other add-on to Thunderbird, but here also occured this error, as well as in Sunbird calendar. I have tried every possible solution given in discussion boards, but nothing could help. It was NOT the firewall, NOT the antivirus and NOT a problem with the profiles (the error occures also on a new installed Firefox/Thunderbird/Sunbird).
A guess of mine was that the Java currently installed led to that installation problem, but also the newest version that was already installed before could not solve it.
Now I am a bit dispaired and pissed off. It seems that no solution is available, nevertheless I hope that one day that stuff will work again (surely only ofter re-building the whole system 
). Until this day I must beg Alex to copy the extensions from his profil to my PC. 
*sigh*
Did you get this sorted? Hopefully this will help someone…
I resolved this problem (eventually) by uninstalling firefox then manually deleting the Mozilla folder which remains under Application Data in my Documents and settings folder.
Hmm, I have tried to delete the Firefox folder etc. but not the Mozilla folder. Maybe this could really be the problem because it occurs also in Thunderbird and Sunbird.
Thanks for this hint.