Firefox for Android only runs a small number of extensions listed on the menu. There is a somewhat complicated procedure to run other add-ons if you install the "Night" Firefox app. More info on that: https://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly/

By the way, why do you want to randomize your browser user agent?

P.S. Do not uninstall the regular Firefox for Android when adding Nightly, they can run side-by-side. If you plan to switch, at least wait until all your data Syncs between the two before removing anything.