Bug 1835625 - Let nightly users set custom param prefs. r=Standard8

Differential Revision: https://phabricator.services.mozilla.com/D179352

toolkit/components/search/SearchEngine.sys.mjs

@@ -4,6 +4,7 @@
 
 /* eslint no-shadow: error, mozilla/no-aArgs: error */
 
+import { AppConstants } from "resource://gre/modules/AppConstants.sys.mjs";
 import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
 
 const lazy = {};
@@ -120,7 +121,12 @@ const ParamPreferenceCache = {
   ]),
 
   initCache() {
-    this.branch = Services.prefs.getDefaultBranch(
+    // Preference params are normally only on the default branch to avoid these being easily changed.
+    // We allow them on the normal branch in nightly builds to make testing easier.
+    let branchFetcher = AppConstants.NIGHTLY_BUILD
+      ? "getBranch"
+      : "getDefaultBranch";
+    this.branch = Services.prefs[branchFetcher](
       lazy.SearchUtils.BROWSER_SEARCH_PREF + "param."
     );
     this.cache = new Map();