Firefox on mobile a few days ago. We developed an extension for client, as part of a solution. Now it doesn't work on most of clients phones anymore, Client is

Firefox on mobile a few days ago. We developed an extension for client, as part of a solution. Now it doesn't work on most of clients phones anymore, Client is quite upset (we told him this might happen, but he is still upset and wants us to fix it, we still haven't found a robust solution that wouldn't be either total rewrite or side loading old unsupported browser)