What if you tried automating Firefox for Android like a regular Android app? As an example for the capabilities:
caps.setCapability(MobileCapabilityType.APP_PACKAGE, "org.mozilla.fennec");
Once Firefox launches, you can interact with it like any other application. The main issue is that Firefox might not connect with a particular driver, so you might not be able to switch contexts or do anything useful.
This is probably not what you wanted to look for.
You might want to post a question in one of the Firefox development forums or IRC as well, asking about Selenium WebDriver automation for Firefox on Android. According to this document, try joining the #ateam IRC on Freenode.
Apparently, @jlipps might be working a bit on this as well. Can you provide us with any guidance? 