The Black Diamond Firstlight is a fantastic bivy tent for particular applications and conditions. It's great for alpine climbing, multi-day ski touring, or any trip where packed size and minimal weight far supersede comfort and livability. The Firstlight is decent for short afternoon thunderstorms but is not ideal in the rain. This model is one of the lightest tents in our review and packs up smaller than a Nalgene bottle (not including the poles). It performs best in dry or below freezing conditions, where all precipitation is frozen. Along with the Black Diamond HiLight, it's also the only tent we tested that's not wholly waterproof.
This tent used to be our favorite bivy tent until the MSR Advance Pro came along. The two weigh about the same, (the Advance Pro is around 1 oz heavier), but the Firstlight is still more packable. The Advance Pro is significantly more weather resistance though, due to its waterproof fabric, and burlier guyline poles. We've given the Pro our Top Pick Award for Lightweight Alpine Climbing, but the Firstlight is still our Top Pick favorite for Lightweight and Packed Size.