After reading numerous answers on StackOverflow, I found that in order to get Splinter to work, I needed to add the geckodriver executable to my PATH variable.
Here's what I did:
In terminal:
echo 'export PATH=$PATH: /Applications/anaconda/pkgs/geckodriver
I then confirmed it was in there via:
echo $PATH
...which shows:
/Applications/anaconda/pkgs/geckodriver:...
Then I tried this (after restarting my Spyder IDE):
from splinter import Browser
browser = Browser('firefox')
...and STILL go this error:
WebDriverException: 'geckodriver' executable needs to be in PATH.
move your geckodriver
binary to /usr/bin/