Enable native events to support typing in rich text areas using WebDriver

See http://code.google.com/p/selenium/issues/detail?id=2331 . The only clean way to make WebDriver type inside a rich text area (contentEditable=true) is to enable the native events. At http://code.google.com/p/selenium/wiki/TipsAndTricks#Enabling_features_that_are_disabled_by_default_in_Firefox they say:

It is disabled by default for Firefox on Linux as it may cause tests which open many windows in parallel to be unreliable. However, native events work quite well otherwise and are essential for some of the new actions of the Advanced User Interaction.

This is the first step required in order to move the WYSIWYG tests to Selenium 2.