Pick through the fir tips, removing any that are damaged or pest-ridden. Remove the papery casings.
Purée the fir tips with a cup or so of vodka in a food processor, blending until fir tips are broken down, about 1-2 minutes.
Pour this mixture into a clean glass jar. Add a bit more vodka to the food processor and blend again for a few seconds to pick up any leftover bits of fir tips. Add to the jar, topping with any remaining vodka.
Put a lid on the jar and give it a good shake. Put the fir tip vodka in the refrigerator and allow to infuse until well-flavored, about a week. Try to remember to shake the jar at least once a day while it's infusing.
Strain the infused vodka into a clean jar using multiple layers of cheesecloth, a jelly bag, butter muslin, or a coffee filter. You can repeat the straining process as needed to remove any solids. Discard the fir tip solids.
Store the fir tip vodka in the freezer until ready to use.