A few days ago, a problematic update for the popular Firefox browser was released. This update caused all plugins to stop working, and they cannot be reactivated. For many users, this plugin issue represents a serious problem. Firefox developers have promised a fix in the coming days.
As a temporary solution, you can install a hotfix from the following address: Hotfix Update.
If you encounter issues installing the hotfix, adjust Firefox settings as follows: Tools - Options - Privacy & Security - Allow Installation of Studies.