BYPASS FIREFOX 48 DISABLING UNSIGNED EXTENSIONS (file paths apply to Windows installation, may be similar on other systems)

1. On a new tab, type about:config, press enter and pay no attention to the warning
2. Search for "xpinstall.signatures.required" and set it to false
3. Search for "xpinstall.whitelist.required" and set it to false
4. Copy this whole text to a Notepad or open it in another browser so you can proceed
5. Exit Firefox
6. Go to C:\Program Files\Mozilla Firefox and create "config.js"  with this content:

//
try {
  Components.utils.import("resource://gre/modules/addons/XPIProvider.jsm", {})
    .eval("SIGNED_TYPES.clear()");
}
catch(ex) {}

7. Go to C:\Program Files\Mozilla Firefox\defaults\pref and create "config-prefs.js"  with this content:

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

8. Start Firefox
9. Install the disabled extensions again, on the top of their disabled local copies, this way they will be enabled again (if it doesn't allow due to the same version number, remove the disabled extension and then install it again)