Intent to unship: element.style.MozAppearance

[Kagami Rosylight]

As of Firefox 143 we removed MozAppearance from the style properties object.

Bug to remove: https://bugzilla.mozilla.org/show_bug.cgi?id=1977489 

This has always been Gecko specific and been used by old websites and libraries to detect Firefox. Removing it fixed a few web compatibility issues as seen in the dependency tree. The actual CSS property `-moz-appearance` is still supported for best compatibility.

(The actual change has made in 2025-07-22, so this is a late notice, sorry. But still in the same cycle!)