Fir vs spruce

The true firs and the spruces are light, pale softwoods often sold together for framing. Red spruce is a little harder and moves a little more than white fir.

Red spruce is about 10% harder than white fir.
Red spruce is about 3% heavier than white fir.
Red spruce has about 12% more movement across a flatsawn board than white fir.

Each column is the species its page leads with: white fir for fir, red spruce for spruce.

Side by side

Property	Fir	Spruce
Janka hardness, 12% MC	480 lbf	530 lbf
Weight, 12% MC	27.3 lb/ft³ (437 kg/m³)	28 lb/ft³ (448 kg/m³)
Specific gravity, 12% MC	0.39	0.40
Tangential shrinkage	7.0%	7.8%
Radial shrinkage	3.3%	3.8%
12 in flatsawn board, 6% → 12% MC	0.176 in	0.197 in
12 in quartersawn board, 6% → 12% MC	0.081 in	0.094 in
Bending strength (MOR), 12% MC	9,800 psi	10,800 psi
Stiffness (MOE), 12% MC	1.5 million psi	1.66 million psi
Crushing strength, 12% MC	5,800 psi	5,540 psi

One source: USDA Forest Products Laboratory, the authority for species properties it measured

Wood Handbook Tables 5-3b, 4-3 and 13-5; weight and movement worked with its Eqs. 4-14 and 13-3. A dash means the Handbook prints no value.

What sets them apart

Telling them apart: The Handbook says the true firs cannot be told apart by their wood alone, and that the eastern spruces (red, white and black) are not distinguished from each other in commerce. (Wood Handbook, p. 2-12 and 2-17)
Strength: The true firs are light and soft; the western firs, except subalpine, are somewhat stronger than balsam fir. The eastern spruces are moderately light, strong, stiff and tough, and dry easily. (Wood Handbook, p. 2-12 and 2-17 to 2-18)
Uses: Fir goes into framing, subflooring, sheathing, boxes and millwork. Spruce goes into framing, millwork, boxes and piano soundboards, and much of it to pulp. (Wood Handbook, p. 2-12 and 2-18)

For any other pair, open each species page from the wood hub; every page uses the same layout and the same sources.