The filter editor is new in Firefox 40.
Clicking the swatch opens a filter editor:
You can add, remove, and edit filters, and drag them to change the order in which they are applied:
MDN will be in maintenance mode for a brief period Monday June 1, from around 3:00 PM until no later than 5:00 PM Pacific (in UTC, Monday June 1, 10:00 PM to 12:00 AM). There should be no disruption in service.