Bug 1997158: Hold CompressionDictionaries in Nightly r=necko-reviewers,sunil

Differential Revision: https://phabricator.services.mozilla.com/D270635

Author: jesup

modules/libpref/init/StaticPrefList.yaml

@@ -15732,7 +15732,7 @@
 # Enable HTTP Compression Dictionary support
 - name: network.http.dictionaries.enable
   type: RelaxedAtomicBool
-  value: true
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
 
 #---------------------------------------------------------------------------