Use API calls to get the labels of all issues and pull requests from the notification list.
The unauthenticated GitHub API is limited to 60 requests per hour, to increase this, you can provide a token to be used.
To get a personal access token go to https://github.com/settings/tokens/new and give the token a name.
To enable private repos, you will need to enable the repos scope for the token.
Open the developer console with F12 or Ctrl+Shift+J and run the following command, substituting YOUR_TOKEN_HERE with your token starting with ghp_.
localStorage.setItem("gh_token", "YOUR_TOKEN_HERE");
Labels are fetched again every 6 hours. To manually clear the cached labels, run the following in the developer console:
localStorage.setItem("labels", "{}");