This has probably a rather small privacy impact, but may prevent some anti patterns (music players stopping when the tab is in the background and similar things). On the other hand it may break some useful features and sites that stop activity when they are inactive will still use CPU (and battery).
Disallow sites to detect if the tab is active (using the visiblity API).
Addon: https://addons.mozilla.org/de/firefox/addon/disable-page-visibility/
This has probably a rather small privacy impact, but may prevent some anti patterns (music players stopping when the tab is in the background and similar things). On the other hand it may break some useful features and sites that stop activity when they are inactive will still use CPU (and battery).