In Firefox for Android Nightly, you can (since recently) sideload extensions via the hidden debug menu (tap the Firefox logo in the About section a dozen times)

In Firefox for Android Nightly, you can (since recently) sideload extensions via the hidden debug menu (tap the Firefox logo in the About section a dozen times). This will add an "Install extension from file" option to the main settings menu. I don't know whether this also works in the standard version of Firefox for Android. It is good they reenabled this feature. Since it is/was very short-sighted of Mozilla to make their centralized addon store the single gatekeeper for extension access on user devices. As if they never thought about power dynamics in software.