Short answers, pulled from the story.
A fir is an evergreen coniferous tree belonging to the genus Abies in the family Pinaceae. There are roughly 48 to 65 living species, found on mountains across much of North and Central America, Eurasia, and North Africa.
Firs have needle-like leaves attached singly to the branch with a circular base set in a small pit in the twig. Their female seed cones stand upright like candles and disintegrate at maturity to release winged seeds, unlike spruces.
Fir trees are typically 40 to 60 metres tall and sometimes approach 100 metres, usually with a single straight trunk. The crown starts conical and becomes more varied in shape as the tree ages.
Caucasian fir, noble fir, Fraser's fir, and balsam fir are generally considered the best Christmas trees. They are valued for aromatic foliage that does not shed many needles as it dries out.
Firs are distributed around the Northern Hemisphere and favour cold climates at high altitude or high latitude. Abies sibirica grows as far north as 67 degrees north in Siberia, while Abies guatemalensis reaches as far south as 15 degrees north in Central America.
Lucas Cranach the Elder painted Madonna under the fir tree for Wroclaw Cathedral in 1510. A copy substituted for the original after the Second World War was discovered partly because the forgery was painted on a fir board while the original was on lime wood, and the original was returned to Poland in 2012.