Some facts:
One does not need Firefox Nightly to run more extensions than the "supported" ones.
Creating a "custom" collection of Add-Ons on addons.mozilla.org can be done using a disposable email address.
There is no visible Settings option to add a custom collection in Fennec from F-Droid. To make it visible, one has to go the About page under Settings and tap the Firefox logo three or more times.
One needs to enter an "ID" in Fennec to add a "custom" collection. The addons.mozilla.org site does not tell the user her ID, it's in the URL that points to her collection.
After a custom collection is added, one loses the default collection of Add-Ons "supported" by Mozilla. No way to easily switch between collections. Need to remove the current collection.
After installing a custom collection, whenever the user access the Add-Ons menu in Fennec, Firefox tries to connect to addons.mozilla.org. Mozilla's definition of user privacy excludes telemetry and data collection by Mozilla.
As another commenter mentioned, many of the Add-Ons that are not "supported" by Mozilla work just fine. They work just as well as uBblock origin, NoScript and the other "supporteed" ones.