The original article only discusses a specific, non-standard (to me, at least – probably made more sense for IT departments) method that Mozilla is calling ‘sideloading’ will be disabled. Per the article, ‘sideloading’ is defined as putting an extension file in a special folder that results in all instances of Firefox on the machine automatically loading that extension.
In the page yorwba linked, it shows how to generate a signed XPI file that can be distributed and installed locally from that file in a more conventional manner (e.g. drag & drop the file onto Firefox’s Add-ons page).
Yes, the older method of extension side-loading, supported at some point by Edge, Chrome, and Firefox, was for IT departments who created OS deployment images with software (incl. extensions) burned into them. Usually this was combined with Group Policy / Device Profile settings that made the extensions impossible to deactivate or remove ("force enabled") and potentially blocked any extensions other than those preinstalled.
I believe that all the browser makers have, at this point, reached a consensus that deploying extensions directly as on-disk files this way makes it 1. too easy for malware to just set itself up as seemingly "deployed by Group Policy"; and 2. makes it too hard for these extensions to be updated as often as they might need to be, or disabled if the browser-maker declares incompatibility with an API in them, etc.
What all the browser-makers seem to favor nowadays, for IT departments who want to do OS-image deployment, is an approach where the burned-in Group Policy will just list out a set of extension IDs that are to be force-installed and force-enabled; and then the browser itself will do the work of retrieving and installing them (but will still treat them like any other extension as it goes through the install process, vetting it for compatibility, upgrading it through its registered upgrade channel, etc.)
This means that every extension in these browsers now has to live in the browser's extension store—even if it's a private, just-for-your-own-company extension. (Which, honestly, there's not much to be said against; the "enterprise deployment" parts of app-stores don't usually force developers to go through pre-vetting before new versions are published or anything. It's just cloud hosting—with the proviso that, due to having download logs, the app-store can see if your "enterprise extension" has an install profile that looks more like that of a virus rather than an enterprise, and then blacklist it.)
Here's Google's documentation on the Group Policy settings that modern Chrome looks for, for comparison: https://support.google.com/chrome/a/answer/7532015?hl=en. Probably Firefox wants to move toward a similar model. And more power to them, honestly; right now, Firefox's extension ecosystem is far too easy a target for malware authors.
In the page yorwba linked, it shows how to generate a signed XPI file that can be distributed and installed locally from that file in a more conventional manner (e.g. drag & drop the file onto Firefox’s Add-ons page).