Fix Bug 1102198, always show unknown plugins. Use subdued style

@alexgibson @pmclanahan  r? 

Unknown plugins are now always shown again, whether it is using old-style plugincheck code or new. Unknown plugins has been removed to the bottom of the list of reported plugins and uses a more subdued style, with icons removed for example.

This also bumps the version of Fx to 34 to ensure new style code does not run in Fx 34 when it is released on 1 December.