Abrowser refuses to install unsigned addons, even after toggling the right flag in about:config

I am developing a WebExtension myself. Right now I am having problem getting my extension signed by Mozilla without having to run Mozilla's nonfree JavaScript[1]. Temporarily, I want to distribute my extension as unsigned and I expect ethical browsers to allow users to install it. Abrowser currently does not (unless as "temporary" extensions but this is not what I want).

Bug experienced on:
abrowser-97.0+build2-0ubuntu0.18.04.1+9.0trisquel85

Steps to reproduce:
* open Abrowser
* go to about:config (click through the warning if one appears)
* search for xpinstall.signatures.required variable and set it to false - from this point I assume Abrowser should allow installation of unsigned extensions
* go to: https://hydrilla.koszko.org/downloads/haketilo-1.0b1.xpi

Expected behavior:
Abrowser should promt the user to install haketilo-1.0b1.xpi

Experienced behavior:
An alert appears with text "Abrowser has prevented this site from installing an unverified add-on." and without any option to proceed regardless.

This is nothing new - since about FF 48 installation of unsigned extensions is disabled in build time of "consumer" builds while for example developer builds are free from this anti-feature. It seems Abrowser is based on the standard edition of Firefox and inherits this bad behavior from it.

[1] https://hydrillabugs.koszko.org/issues/111