Firefox 136 supports webextensions.manifest.background.preferred_environment (#25808)

* Firefox support version fou webextensions.manlfest.background.preferred_environment (Bug-1930334)

* Added quotes

Author: rebloor

webextensions/manifest/background.json

@@ -109,7 +109,7 @@
               },
               "edge": "mirror",
               "firefox": {
-                "version_added": false
+                "version_added": "136"
               },
               "firefox_android": "mirror",
               "opera": "mirror",