Bug 1901497 - Enable -moz- transform prefixes on nightly again. r=webcompat-reviewers,twisniewski

This changes the approach of zoom+moz-transform compat on Nightly, on
the basis that its easier to site-patch stuff that uses both
-moz-transform and zoom (to disable either of them via CSS) than working
around bogus UA sniffing or feature detection that is sorta-reasonable
(like assuming that if "MozTransform" is in the CSSStyleDeclaration
object, then `-moz-transform` works).

If breakage from this is too severe, or the hypothesis above doesn't
hold, then we need to re-evaluate this in some other way.

Differential Revision: https://phabricator.services.mozilla.com/D216840

modules/libpref/init/StaticPrefList.yaml

@@ -9178,7 +9178,7 @@
 # Is support for -moz-prefixed transform properties enabled?
 - name: layout.css.prefixes.transforms
   type: bool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 # Is support for -moz-prefixed transition properties enabled?