Unfortunately there is nothing I can do about that. Mozilla dropped add-on support in Fenix except for some hand picked ones. CanvasBlocker is not part of that list (it is not "recommended" by Mozilla).

It's up to Mozilla to decide on add-ons in Fenix: https://blog.mozilla.org/addons/2020/09/02/update-on-extension-support-in-the-new-firefox-for-android/