Answer: You can usually cut off one root without damaging a fir tree. They have many anchor roots but most are not above ground level so I don't know whether there are underground roots that might be worrysome or perhaps threatening the foundation of your house. If you are concerned you might have an arborist come out and inspect the tree. After looking at the tree and exploring the ground with probes, he can tell you what potential there might be for damage. You can then decide whether or not you want to keep the tree. Best wishes with your landscape.
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