There was a small problem. I use portable firefox 60.0.1 at home and at work. They have your extension installed. When you start the browser at work and when the notification window for past events appears, firefox closes without any error messages. And this happens if you do not have time to close the notification window manually and then not always. I do not see such problems on the PC at home. Perhaps it's worthwhile to make a timeout for displaying notifications right after firefox is enabled?