Explain how to allow Firefox to share screen

[dani]

Since a few version (32 ?) Firefox can share individual windows just like Chrome can with an extension. The difference is that Firefox do not need an extension but a setting in about:config (media.getusermedia.screensharing.allowed_domains).

If permission is denied, a modal dialog should be opened and explain how to do this, just like we show a dialog asking to install the extension for Chrome users