In a blender, add ¼ cup of the heavy cream and the packet of gelatin. Set aside to let the gelatin soften while you prepare the rest of the recipe.
In a medium saucepan, heat the remaining 1¾ cups heavy cream with the sugar and salt until bubbling, but do not let it boil. Remove from heat and add this mixture to the blender. Blend on low for 30 seconds, then add the fir tips. Blend again until the tips are incorporated and the mixture is a light green color, about 1 minute.
Pour the mixture through a fine-mesh sieve to strain out the tips. Whisk in the lime juice.
Thinly coat four (6-ounce) ramekins with vegetable oil. Divide the cream mixture evenly between the ramekins and refrigerate for 2–4 hours.
Once set, turn out the panna cotta by inverting each ramekin onto a serving plate (if desired) and garnish with more fir tips.
Note: These are best served the same day you make them. Cover any panna cotta you won’t be eating the same day. They will become firmer the longer they sit but will still be delicious. Best eaten within 2–3 days.