Hi there,
I can build this extension for you. Let me know what are the tasks that can be done using the plugin and which website should it "communicate" with. Interested and looking forward to your reply. I may lower the price if the task is not too complicated.
The plugin will use local storage as a database for offline work, and we may need an API on your side on the server for syncing the data (depends).
Forgot to mention, I can also provide a Firefox extension at no extra cost.
Regards,
Julijan