Programmatically adjust policy preferences (enable file:///// urls)

We have an intranet website/application which has been optimized for Firefox, so every user using this application is using Firefox. We desperately need a new feature where users can open a document from a network share in such a way that they can edit it, save it and then close it again. This would normally be done with a href="file://///servershare/file" link, except some time ago this has been disabled by default in firefox.
As i understand, this can be enabled by adjusting the policies in user.js - this is however not an option. The people that use the application have very little technical knowledge and this would be too complicated to explain to every single user.
Unfortunately we also don’t have an infrastructure where this can be pushed via a policy.

So, i need to be able to solve this from either within the application or by means of an add-on/extension. My guess is that from within the application (a PHP generated HTML application) is not a possibility because that would mean i could circumvent the security model. So i’ve looked at building an extension. I’ve found examples how to do this in the old way of making extensions, but since these are going to be phased out by the end of this year (2017), that is not a possibility either. WebExtensions seem the way forward, but sadly enough it seems they don’t support adjusting these policies: https://wiki.mozilla.org/WebExtensions/FAQ#Will_I_have_access_to_about:config_or_the_preferences.3F

So what are my options here?
Any insights are much appreciated.

This question seems more appropriate for the Enterprise group:

https://wiki.mozilla.org/Enterprise

WebExtensions probably won’t support many enterprise-specific cases, so any gaps should be covered with other tools.