Unable to install new / update Firefox Plugins - Fixed

After upgrading from Firefox 2.x to Firefox I found myself unable to install new plugins. It would keep on telling me that it would install the plugin at the next restart of Firefox.
Error: When I opened the Error Console it would say:

[Exception... "Component returned failure code: 0x80520012 (NS_ERROR_FILE_NOT_FOUND) 
nsILocalFile.persistentDescriptor" nsresult: "0x80520012 (NS_ERROR_FILE_NOT_FOUND)" 
location: "JS frame file:///Applications/Firefox/Firefox.app/Contents/MacOS/components/nsExtensionManager.js
getAbsoluteDescriptor :: line 438" data: no]

Solution:

close firefox
goto your profile on $HOME/Library/Application Support/Firefox/Profiles/<your profile-id>
rm extension.ini ; rm extension.rdf ; rm extension.cache
restart firefox

See also the mozilla knowledge base: http://kb.mozillazine.org/Unable_to_install_themes_or_extensions_(Firefox)#Corrupt_extension_files