Hi To install a complete add-on, you are best to head over to the Firefox for Android add-on site from within Firefox for Android. If you are a developer making your own add-ons, I recommend you have a read of this [https://developer.mozilla.org/en-US/Add-ons page] which should point you in the right direction. I hope this helps, but if not, please come back here and we can look into a different solution for you.