In your browser, install TamperMonkey or another Userscript manager browser extension.
Then you can simply go to https://gist.github.com/marnix/0d27e891be359afc69ba5b090b6607b1/raw/jira-issue-page-updated-notification.user.js, which should trigger the browser extension to install the latest version of this userscript. And you can also use the same URL for updating to newer versions (which I occasionally expect to release).
An identical copy is also published via Greasy Fork (https://greasyfork.org/en/scripts/462479-jira-issue-page-updated-notification).