Don't use a NPAPI plugin for Firefox: access the system via ctypes
Take the account-creation code out of the NPAPI plugin into its own tiny shared library (libwebaccounts.so); the NPAPI, still used by Chromium, has been modified to link to this library, while for Firefox we'll be using ctypes (https://developer.mozilla.org/en-US/docs/Mozilla/js-ctypes?redirectlocale=en-US&redirectslug=js-ctypes) to interface with the new library, of which we use just one function.
In this way we get also rid of a couple of bugs related to the embedding of the NPAPI plugin.