No Addon Webextension XPI install - Atavic/legacy-user.js Wiki

How to disable Addon installation

Configure Firefox to not allow the installation of any addon (plugins and webextensions). The restriction will apply to all users of the computer system. You will need administrator access on the computer to save the configuration.

With this setting enabled if a user tries to install an add-on they will see this message: Software installation has been disabled by your system administrator.

Close Firefox. Create a new file ‘local-settings.js’ and put these lines:

pref("general.config.obscure_value", 0);
pref("general.config.filename", "mozilla.cfg");

Create another file ‘mozilla.cfg’ and put these lines:

//Firefox Default Settings
//Disable add-on installation
lockPref("xpinstall.enabled", false); 

Save the files to the following folders:

Windows 32-bit
    local-settings.js  –   C:\Program Files\Mozilla Firefox\defaults\pref
    mozilla.cfg           –  C:\Program Files\Mozilla Firefox

Windows 64-bit
    local-settings.js  –   C:\Program Files (x86)\Mozilla Firefox\defaults\pref
    mozilla.cfg           –  C:\Program Files (x86)\Mozilla Firefox

Try installing an addon or webxtension. You should receive the aforementioned error message.