I use Firefox 40.0.3 from Fedora 21 updates repository

$ rpm -q firefox

Every PKI CA certificate profile containing keyGenInputImpl input produces warning message on a web page:

Warning: This version of Firefox no longer supports the crypto web object used to generate and archive keys from the browser. As a result expect limited functionality in this area.

But actually key generation works in my Firefox version.

What does this warning really mean?