

JavaScript is already sandboxed. You can only execute functions where there is an actual API defined by the browser to do so, for example Date.getTime()
. There is / should be no way to get, say, your device ID. (With the exception of unpatched exploits that allow executing arbitrary code. But keep in mind browsers are likely one of the if not the most security tested software.)
What you linked to here appears to specific to Google Tag Manager in a way that I don’t fully understand, but is not related to how websites usually execute JavaScript code.
That probably counts as a privileged page, as in something uBlock Origin can’t access or modify.
Mozilla has probably been running another “experiment”, meaning not every user is affected. In the past they claimed it’s not advertisements because they are “continually looking for more ways to say thanks for using Firefox”. (Bullshit.) If you go to Settings > Home, you disable anything you don’t want to see, or just set your home page to a blank page, period.