Bug 1733340 - Disable backdrop-filter for now. r=jnicol

There are a fair amount of performance and correctness issues that need
to be worked on, and at least I don't have the time to work on them
right now, so let's disable it until some of the issues blocking
bug 1578503 are fixed.

Differential Revision: https://phabricator.services.mozilla.com/D127085

modules/libpref/init/StaticPrefList.yaml

@@ -6836,7 +6836,7 @@
 # Is support for CSS backdrop-filter enabled?
 - name: layout.css.backdrop-filter.enabled
   type: bool
-  value: @IS_EARLY_BETA_OR_EARLIER@
+  value: false
   mirror: always
 
 # Should stray control characters be rendered visibly?