Comment by 13of40

It seems like the two spellings were used interchangeably until about 1960:

https://books.google.com/ngrams/graph?content=referer%2C+ref...

Edit: Actually, that doesn't seem to be true -- although there's some usage of "referer" in English, most of the hits in Google Books prior to 1960 turn out to be for the Old French word "referer".