Hi,

The extension is activated for all websites which contain "owa" in the url path. More precise, the regex used is: "*://*/owa/*"
This is so because the extension works not only for office365 hosted by Microsoft (https://outlook.live.com/owa/) but also with internal hosted OWA by any company. Also for the enterprise office365 some companies use their own domain.

But if you have any security related concerns, you can build a custom extension just for your domain. The code is here: https://github.com/mihai-chezan/owa_notifications_firefox_extension
You just have to change the manifest.json, the line with ""matches": ["*://*/owa/*"]," and replace it with your domain.