Can you use Not So Fast to counter Brynhir Thundersong's ability?

Ruling: No, you cannot use Not So Fast to counter Brynhir Thundersong's ability. Not So Fast can only counter spells or abilities that choose a friendly unit or gear, and Brynhir Thundersong's play effect is a global effect that does not target or choose a specific unit or gear.

Nuances:

• You can respond to Brynhir Thundersong's ability with other reactions (such as activating a hidden Zonyah's) while her "When you play me" trigger is still on the chain
• Once her ability resolves, you will be prevented from playing any further cards for the remainder of the turn