This is this commit, which should land soon on Nightly.

Do you have the Nightly version?

From a discussion on Reddit, it appears that the Nightly version supporting addon collections isn't available on Google Play at the time of writing, but you can get the APK directly:

https://firefox-ci-tc.services.mozilla.com/tasks/index/mobile.v2.fenix.nightly.latest

It will be nice to have it on Play Store too, but good to see that:

1) This feature is now available

2) Mozilla is once again accommodating users who can't or don't wish to use Play Store

This functionality is now available on the Firefox Nightly that comes from Google Play too!

Instructions on how to use it: https://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly/

What those instructions don't mention explicitly is that after doing all the steps they describe, you still have to actually enable the extensions! You do that in the mobile app menu from Add-ons >> Add-ons manager, just like you would for the "standard" Firefox Mobile extensions.