Thanks for asking about plants that will grow in the dry shade under your fir tree. There are quite a few plants that are adapted to such conditions, but it can be challenging getting them started - the tree roots are already taking up all the available water and nutrients, and if they are very dense, it can be hard to even dig a hole to plant new small plants. You may have to do some experimenting to find out what will grow well. It is best to start with small plants; dig them as much of a hole as you can, and water them regularly to help get them established (of course, the tree roots will also grow into the amended, watered soil, so eventually they will have to compete with those). Another way is to put a thin layer (no more than 3-4") of good soil over the whole area, and plant your new plants in that. Again, the tree roots will fill it up quickly, but this gives the small plants a head start.
As to what plants can grow in dry shade, quite a few NW natives are adapted to that habitat, although many of them survive by going dormant in the dry summer. Some to try include:
Smilacina racemosa (false Solomon's-seal)
Symphyotrichum (Aster) subspicatus (Douglas aster)
Tellima grandiflora (Fringecups)
Polystichum munitum (sword fern)
Gaultheria shallon (Salal)
Mahonia aquifolium (tall Oregon grape)
Mahonia repens (creeping Oregon grape)
Rosa gymnocarpa (bald hip rose)
Symphoricarpos albus (snowberry)
Dicentra oregana (western bleeding heart)
Fragaria vesca (Wood strawberry)
Oxalis oregana (wood sorrel)
There are non-natives as well that can handle dry shade. Here is a link to one good list from UMN Extension: https://www.extension.umn.edu/garden/yard-garden/landscaping/best-plants-for-tough-sites/
- look on the list to the right for dry and shade lists. Euphorbia amygdaloides var. robbiae is an attractive, near-evergreen euphorbia that shouldn't be grown anywhere except dry shade - it is too vigorous a spreader anywhere else. Hellebores are also good evergreen candidates.
If you try several different plants you are pretty sure to find something that will grow well for you. Good luck!