This feature is built in to the core of the extension. You don’t have to do anything beyond having the extension installed and being logged into.
This information is provided via an X-Ray line and as a gray or green dot next to the chat row on the left sidebar. You can see in the image below.
The online & offline status is delayed by a 30 minute window. If you see a green bubble, it does not mean that a user is online right this moment. However it does suggest that they have opened the app in the last 30 minutes.
If you would rather not see the status via X-Ray line, you can click on the extension icon to open settings
Then choose the Options tab and uncheck the X-ray toggle.
Note that this will also hide the information on Tinder, where it shows how long a user has been on Tinder for (i.e. when they have registered for).