Add alert for all the Firefox users once the add-on is fixed

Created by: AlicjaSuska

Part of the RFC 221: Improve discoverability/awareness of the browser extension

Description Once the Firefox extension is back, we need to communicate this to users. It's a great opportunity to highlight the benefits of using the extension as well.

Design documentation

• display this alert for all the users the day Firefox add-on is back
• as the regular alert, it should be displayed in the context of a file
• after 4 weeks, display the regular alert instead
• Add an event to the link in the alert to get data on how many people clicked on it @ebrodymoore FYI (for cloud only; server pings coming soon)