Starting with Firefox 48, Mozilla made the add-on signature enforcement mandatory. The user cannot disable it using the about:config flag or using any other method. Here is a hack which will allow you to bypass the requirement and install unsigned addons in the browser.
As you may already know, Firefox 44 and above won't allow you to run unsigned add-ons. Mozilla developers decided to remove the ability to disable the signature enforcement, which can still be disabled in Firefox 43. However, it was announced that the stable branch of Firefox will get a new option which will allow the user to temporarily load unsigned extensions.
As I installed Firefox 43 on all my PCs, it disabled some add-ons on my laptop. This is because of the new requirement that all addons be signed by Mozilla. If any of your favorite add-ons stopped working because of this signature enforcement added to Firefox 43, here is what you can do.