There are a lot of recent negative reviews on that add-on complaining of the same issue. Unfortunately, since add-ons are developed by third-party developers, there's nothing that Mozilla can do.

You will need to wait for the developer to fix the issue. If possible, you should report the issue to the developer directly using their contact information so that they can address the issue.

Thank you for your response. However, I'm not sure that in this case the add-on is itself the problem. The add-on has not been altered recently, and was working normally. Firefox has been updated recently. On the other hand, all my other add-ons (very few) are still working. Do you know of any changes in the recent Firefox updates that may have impacted this? The additional security functions that have been added seem to be a logical place for a conflict. Thanks for your thoughts.

I'm not aware of any changes that would cause the add-on to break. Unfortunately, it's the add-on developer's responsibility to make sure that their add-on works on new versions of Firefox and fix any issues that prevents it from working properly.

Issues with specific add-ons (especially if they are reported by multiple users), generally come down to an issue with the add-on's code. This is an issue that the developer needs to address directly and falls outside the scope of this community.