...Just for anyone who comes looking later and might be misled by the previous comment:
According to several different print dictionaries, that's backwards.
Furring with U is correct and firring with I is a misspelling or sometimes described as an alternate spelling.
It goes back before European settlement of America and naming of the Douglas Fir. It might have been named for another fir in England, but it wasn't. It was probably named for a similar technique used to change the dimensions of misbuilt wooden ships.