In Firefox Android's recent update extensions support has been decreased. You can read more here https://blog.mozilla.org/addons/2021/01/20/extensions-in-firefox-for-android-update/

But still you can use extensions on Firefox Nightly for Android(https://play.google.com/store/apps/details?id=org.mozilla.fenix&hl=en&gl=US). It's developer version of Firefox for android.

Follow the article https://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly/ to set up.