CSS nesting support
With firefox 117, CSS nesting has been enabled. Mozilla meta bug.
It is not enabled in ESR 115. Whilst there is layout.css.nesting.enabled
to enable it, a lot of the linked bugs aren't being uplifted to esr115, so I'm not sure it will be stable. Since all major browsers now mostly support it, there's a chance that some web pages would start using it, with the result being broken CSS for tor browser and mullvad browser (for firefox ESR 115 as well). I'm not really experienced enough to know how likely this would be though.
When we get this at the next ESR, we should be aware that firefox has started porting some of its internal stylesheets to use nesting, so this might break some of our patches on next rebase.
And once we get this, we can start using it for our own components as well