Not sure who still looks at this list - hopefully some people still :)
I wanted to let you know that the beta 53 for Firefox for Android that just started shipping on the Play Store, now has 4 locales included along with RTL support:
https://play.google.com/store/apps/details?id=org.mozilla.firefox_beta
Keep in mind you that at the time, you need to use the device's language switcher as well in order in order to get RTL support (so not only from Firefox's locale switcher).
So switching your device *and* you Firefox for Android to either Persian, Urdu, Hebrew or Arabic should do the trick :)
Thanks to these teams for all their hard work on this!
Delphine