Support Firefox WebDriver
Or maybe it's just called Gecko.
Installing the Chrom(ium) WebDriver is not possible or practical on some platforms. In !23 (merged), we already made some progress towards more flexibility in drivers, let's build on that further.
The primarily goal would not be to test that the front part of Heptapod works with Firefox, because at the moment, we don't have specific JavaScript code and almost not styling, hence any problem is upstream. Still it'd be good to know.