Install a personal firefox web extension permanently

Question

Previously, I could write an addon for personal usage packed as something.xpi and I clicked on it to install it.

After a while, mozilla introduced xpi

Answer 1

What you should be looking for is having your extension signed by Mozilla as Unlisted.

See Mixing Listed and Unlisted Add-ons on addons.mozilla.org blog post for an overview.

That way, AMO does not host nor (normally) review your extension; it simply runs some basic automated checks and immediately signs your extension so that it can be privately distributed as an XPI.