Thanks for writing. It's difficult gardening under established evergreen trees because they cast too much shade for lawngrasses to grow. Instead, you'll want to look for shade-loving groundcovers.
But the problem with the firs is that they have massive root systems -- and they absorb the lion's share of the moisture and nutrients from the soil. So you'll need to give anything you plant under your firs extra water to get them established.
Some relatively drought-tolerant groundcovers that like shade include epimedium, lily-of-the-valley, and bigroot geranium. Learn more about these and other good varieties in our Plant Encyclopedia! http://www.bhg.com/gardening/plant-dictionary/