Firefox has three APIs: XUL, Jetpack, and WebExtensions. XUL has been on the way out for years, and never worked in mobile Firefox anyway. (Almost nobody runs mobile Firefox, so that turned out not to matter much.) Jetpack was introduced in 2013, and by 2014, it was actually usable. It's not clear when and whether Jetpack add-ons will stop working. Mozilla's developer documentation is so confused that you can't tell.…
FWIW, I love mobile Firefox and I really don't understand why folks use Chrome instead.