I just finished writing an article that explains how to connect WebDriver to Firefox running in the new headless mode on Windows if anybody's interested: https://intoli.com/blog/running-selenium-with-headless-firef... It should be pretty similar on Linux, and probably macOS when it comes around.
> "c:\Program Files\Nightly\firefox.exe" -headless
(It also works when Firefox is invoked from a Node.js script in a test suite for a Node.js program that drives Firefox.)
Perhaps this is a Selenium issue?