a) The Mozilla must have the configuration option
“signed.applets.codebase_principal_support” set to the value “true”.
This allows non-signed scripts to request higher privileges.
b) Selenium must request higher privileges which can be handled in
different places. To allow typing into file fields you can include this
in the file selenium-api.js in function Selenium.prototype.doType. This
enables uploading local files.
This workaround works also with Firefox.