Bug 1874689 - Enable forkserver by default in Nightly. r=jld

Differential Revision: https://phabricator.services.mozilla.com/D198533

dom/docs/ipc/process_model.rst

@@ -296,7 +296,7 @@ Fork Server
 -----------
 
 :platform: Linux only
-:pref: ``dom.ipc.forkserver.enable`` (disabled by default)
+:pref: ``dom.ipc.forkserver.enable`` (enabled by default in Nightly)
 :primary protocol: *none*
 :sandboxed?: no (processes forked by the fork server are sandboxed)
 

modules/libpref/init/StaticPrefList.yaml

@@ -2821,7 +2821,7 @@
 #ifdef MOZ_ENABLE_FORKSERVER
 - name: dom.ipc.forkserver.enable
   type: bool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: once
 #endif