From github: https://github.com/mozilla-mobile/fenix/issues/18135.

What is the user problem or growth opportunity you want to see solved?

It is impossible to install unlisted Web Extensions in Firefox Nightly.

Currently, in order to use a simple custom extension in Nightly, one has to:

1. Create a Firefox Account;
2. Create a collection;
3. Upload the extension to be listed at addons.mozilla.org (unlisted extensions CANNOT be installed);
4. Waste the time of the reviewer to look at the extension;
5. Pollute the add-on database with an extension that is of very little use to the general public.

Curiously, issue #11308, which raised roughly the same issue, was closed as a duplicate of #14034, which is now closed and did not address this specific problem.

┆Issue is synchronized with this Jira Task

Change performed by the Move to Bugzilla add-on.