Use Yandex.Mail notifier instead, it's based on WebExtension API which supported by Chrome, Firefox, Opera etc.
Tunable Firefox extension for Yandex.Mail with lots of features.
- No need to enter your credentials
- Handles emails from all folders
- English and Russian localizations
- Tunable desktop and sound notifications for new and unread emails
- Panel for quick work with emails
- Indicator and notifications when you're not authorized
The addon is built with JPM.
Addon page: https://addons.mozilla.org/addon/yandex-mail-notifier