We used to have a somewhat eccentric music teacher in Liverpool who insisted that the books all got it wrong and it should be "fir" tree - so that's the way our class learned it.
I was always rather taken with the "perdrix" explanation
Jim Carroll
"Twelve days of Christmas" was adapted from similar New Years' or spring French carols, of which at least three are known, all featuring a partridge, perdriz or perdriole, as the first gift. The pear tree appears in only the English version, but this could also indicate a French origin. According to Iona and Peter Opie, the red-legged (or French) partridge perches in trees more frequently than the native common (or grey) partridge and was not successfully introduced into England until about 1770. Cecil Sharp observed that "from the constancy in English, French, and Languedoc versions of the 'merry little partridge,' I suspect that 'pear-tree' is really perdrix (Old French pertriz) carried into England"; and "juniper tree" in some English versions may have been "joli perdrix," [pretty partridge]. Sharp also suggests the adjective "French" in "three French hens", probably simply means "foreign".

In the northern counties of England, the song was often called the "Ten Days of Christmas", as there were only ten gifts. It was also known in Somerset, Dorsetshire, and elsewhere in England. The kinds of gifts vary in a number of the versions, some of them becoming alliterative tongue-twisters. "The Twelve Days of Christmas" was also widely popular in the United States and Canada. It is mentioned in the section on "Chain Songs" in Stith Thompson's Motif-Index of Folk-Literature (Indiana University Studies, Vol. 5, 1935), p. 416.