Craig Conley sends this funny photo along, along with the observation they might want to consider the wording on the bumper sticker a bit... further.
Deciding whether to use farther or further can be tricky. Generally, we use "farther" with distance--both literal and figurative, and further when we're talking about degrees. So, you drive your minivan farther down the highway, but you discuss things further when you're stuck in traffic.
According to James Kilpatrick, "farther" is a typographical error. The original word, he says, is further.
Most people intuitively know when to use farther and when to use further. No one says, "Let's discuss this farther." Likewise, "farthermore" isn't a word you hear every day, and the only people who say, "It isn't fur from here" are cartoon roosters (either that, or surprised mink trappers).
When in doubt, "further" is probably the safest choice, because you can always say it's the older word.