I'm still trying to get the monkey to load a script in Firefox for Android (Fennec was a nicely shorter title), but I agree with @JonnyOne - why does GitHub have newer releases than AMO?

Regardless, will this Android Firefox allow me to install the newer versions from GitHub like my desktop Firefox did?

Indeed! I was able to install the newer xpi's from GitHub on my Paleophone (Note3, rooted so stuck to 4.4.2)