Just a moment ago, I wanted to install some Firefox extensions and update some others, but every time I tried FF gave this error:
“Firefox could not install the file because: Unexpected installation error. Review the error Console log for more detail. -203″
The fix has been quite easy. First result on a Google search was this page, where they explain a method that seems quite a drag (basically, whipe all the config out and reinstall everything), but some guy suggests in a comment that the only file you need to remove is extensions.rdf. It worked for me, just by deleting that file the problem was gone. 
By the way, on MacOS X this file is in:
$HOME/Library/Application Support/Firefox/Profiles/PROFILE/extensions.rdf.
donde encuentro ese fichero
Depende de tu sistema operativo. En Mac donde pone en el artículo, en Linux en $HOME/.mozilla/blablabla (o .firefox, no tengo uno ahora a mano) y en Windows supongo que en “Documents and Settings/Mozilla/Firefox/algo”.
hi, noticed this post through back links. The reason why some profiles need extension-reinstal & re-config is that it is not ALWAYS the case that a single file is corrupt. Also withe extensions like FEBE, recovering your customized profile is simple
En este foro hay una respuesta práctica para los problemas con el error 203 y los addons o plugins de firefox que no funcionan (para Windows)
http://www.mozillaes.org/foros/viewtopic.php?f=14&t=26722&p=137836#p137835