Read Aloud might be a serious add-on for desktop Firefox, but, frankly, it's implementation in Firefox for Android 🦊 is pretty lousy if not useless:
1) There is only one very robot-ish (female) voice per language!
2) The UI within Firefox is more than pathetic!
3) There are no Android notification center controls!
4) When quitting Firefox, it just continues reading aloud!

In contrast, Mozilla's Pocket for Android is really convincing with a choice of ~10 great TTS voices per language, sounding very natural & pleasant like a radio announcer.

IOW: back to the workbench, developers!