How to force install an unverified firefox extension in 41.0b1+?

Yes, there is a setting in About:config, its name is xpinstall.signatures.required. Double-click on the preference name so that its value is set to false. Now you can install unsigned extensions in Firefox. A browser restart is not needed.

On Firefox 48 and above, this method doesn't work. Instead you should create two config files inside Firefox directory.

1. Create config.js file in notepad (make sure file extension is .js and not .txt):

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

2. Move config.js to your Firefox installation directory:

   Windows: C:\Program Files\Mozilla Firefox
   (or C:\Program Files (x86)\Mozilla Firefox )

   Linux: /usr/lib64/firefox-< version >
   (or /usr/lib/firefox-< version > )

   Mac: /Applications/Firefox.app

3. Create config-prefs.js in notepad:

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

4. Move config-prefs.js into Firefox defaults\pref directory.
   (e.g. C:\Program Files\Mozilla Firefox\defaults\pref)

5. Restart Firefox.

6. Drag the unsigned XPI into Firefox window, or use the "Install Add-on From File" option, in the settings of Firefox Add-ons.

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Originate from this source: https://forum.mozilla-russia.org/viewtopic.php?id=70326