Add a test for the handling of Firefox preferences
Add a test for the handling of Firefox preferences

This test only works with a new version of Firefox and GeckoDriver
directly. Selenium 2 doesn't seem to do the right thing and Selenium 3
has a bug causing it not to support setting Firefox capabilities.

tebeka/selenium,RaviTezu/selenium,tebeka/selenium,RaviTezu/selenium,RaviTezu/selenium,tebeka/selenium