I found it, I did it and it's working ! (just remember that in the end you have to reinstall the unsigned add-on/s you really want to keep )
-- Mozilla implemented the add-on signing security feature in Firefox some time ago, and made it mandatory in Firefox 48 for the Stable and Beta channel.
This means that Firefox users who run Stable or Beta versions of the browser can only install signed add-ons in the browser.
Firefox Dev, Nightly and ESR users can install unsigned add-ons however after making a change in the preferences.
Add-on developers get access to unbranded Firefox builds which are stable versions of Firefox that still support the preference.
All add-ons submitted to Mozilla's main add-on repository are signed, but that is not necessarily the case for add-ons distributed through other channels.
Add-ons provided by applications, security software comes to mind, on development platforms like Github, and old add-ons that are not in development anymore, may not be signed. These add-ons cannot be installed in Firefox Stable or Beta in that case.
The method requires that you create two new files and place them in the main Firefox installation / run folder.
Step 1: Create config.js file
firefox override add-on signing
Create a config.js file first. You can do so on Windows by right-clicking on the desktop and selecting New > Text Document.
Leave the name for the time being, and open the text document afterwards by double-clicking on it.
Paste the following code into the document:
Save the document afterwards.
Right-click on it, and select rename.
Name it config.js. Make sure it is called config.js and not config.js.txt.
Move the config.js file in the Firefox installation folder.
On Windows, this is either C:\Program Files\Mozilla Firefox\
or C:\Program Files (x86)\Mozilla Firefox.
On Linux, it is /usr/lib/firefox- or /usr/lib64/firefox-
On Mac, it is /Applications/Firefox.app
Please note that you are free to select any installation directory, and that you may run Firefox as a portable program as well. Adjust the program path accordingly. Make sure you place the file in the root program folder of Firefox.
Step 2: Create config-prefs.js
firefox bypass add-on signing
Create a second text document on the desktop and open it afterwards in an editor.
Paste the following lines into it:
Rename the file to config-prefs.js.
Move the file into the defaults\pref folder of the Firefox program folder, e.g. C:\Program Files\Mozilla Firefox\defaults\pref
Step 3: Install unsigned add-ons in Firefox Stable or Beta.
Once done, you may once again install any add-on, signed or unsigned, in Firefox Stable or Beta just like before. --
(font: http://www.ghacks.net/2016/08/14/override-firefox-add-on-signing-requirement/ )