diff --git a/netwerk/protocol/http/nsHttpHandler.cpp b/netwerk/protocol/http/nsHttpHandler.cpp
index da2c5fa31cfa..207abc2228d8 100644
--- a/netwerk/protocol/http/nsHttpHandler.cpp
+++ b/netwerk/protocol/http/nsHttpHandler.cpp
@@ -99,9 +99,7 @@
 #  include "nsCocoaFeatures.h"
 #endif
 
-#if defined(MOZ_BUILD_APP_IS_BROWSER) && !defined(ANDROID)
-#  include "mozilla/browser/NimbusFeatures.h"
-#endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
+#include "mozilla/browser/NimbusFeatures.h"
 
 //-----------------------------------------------------------------------------
 #include "mozilla/net/HttpChannelChild.h"
@@ -129,10 +127,8 @@
 #define NS_HTTP_PROTOCOL_FLAGS \
   (URI_STD | ALLOWS_PROXY | ALLOWS_PROXY_HTTP | URI_LOADABLE_BY_ANYONE)
 
-#if defined(MOZ_BUILD_APP_IS_BROWSER) && !defined(ANDROID)
-#  define UA_EXPERIMENT_NAME "firefox100"_ns
-#  define UA_EXPERIMENT_VAR "firefoxVersion"_ns
-#endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
+#define UA_EXPERIMENT_NAME "firefox100"_ns
+#define UA_EXPERIMENT_VAR "firefoxVersion"_ns
 
 //-----------------------------------------------------------------------------
 
@@ -142,7 +138,6 @@ namespace mozilla::net {
 
 LazyLogModule gHttpLog("nsHttp");
 
-#if defined(MOZ_BUILD_APP_IS_BROWSER) && !defined(ANDROID)
 static void ExperimentUserAgentUpdated(const char* /* aNimbusPref */,
                                        void* aUserData) {
   MOZ_ASSERT(aUserData != nullptr);
@@ -176,7 +171,6 @@ static void ExperimentUserAgentUpdated(const char* /* aNimbusPref */,
   aExperimentUserAgent->Truncate();
   aExperimentUserAgent->AppendPrintf(uaFormat, firefoxVersion, firefoxVersion);
 }
-#endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
 
 #ifdef ANDROID
 static nsCString GetDeviceModelId() {
@@ -279,9 +273,7 @@ nsHttpHandler::nsHttpHandler()
 
   mUserAgentOverride.SetIsVoid(true);
 
-#if defined(MOZ_BUILD_APP_IS_BROWSER) && !defined(ANDROID)
   mExperimentUserAgent.SetIsVoid(true);
-#endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
 
   MOZ_ASSERT(!gHttpHandler, "HTTP handler already created!");
 
@@ -389,14 +381,12 @@ nsresult nsHttpHandler::Init() {
                                        gCallbackPrefs, this);
   PrefsChanged(nullptr);
 
-#if defined(MOZ_BUILD_APP_IS_BROWSER) && !defined(ANDROID)
   // monitor Firefox Version Experiment enrollment
   NimbusFeatures::OnUpdate(UA_EXPERIMENT_NAME, UA_EXPERIMENT_VAR,
                            ExperimentUserAgentUpdated, &mExperimentUserAgent);
 
   // Load the experiment state once for startup
   ExperimentUserAgentUpdated("", &mExperimentUserAgent);
-#endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
 
   Telemetry::ScalarSet(Telemetry::ScalarID::NETWORKING_HTTP3_ENABLED,
                        mHttp3Enabled);
@@ -759,13 +749,11 @@ const nsCString& nsHttpHandler::UserAgent() {
     return mUserAgentOverride;
   }
 
-#if defined(MOZ_BUILD_APP_IS_BROWSER) && !defined(ANDROID)
   if (!mExperimentUserAgent.IsVoid()) {
     LOG(("using Firefox 100 Experiment User-Agent : %s\n",
          mExperimentUserAgent.get()));
     return mExperimentUserAgent;
   }
-#endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
 
   if (mUserAgentIsDirty) {
     BuildUserAgent();
diff --git a/netwerk/protocol/http/nsHttpHandler.h b/netwerk/protocol/http/nsHttpHandler.h
index 21e752b620b5..313de72be1c9 100644
--- a/netwerk/protocol/http/nsHttpHandler.h
+++ b/netwerk/protocol/http/nsHttpHandler.h
@@ -648,9 +648,7 @@ class nsHttpHandler final : public nsIHttpProtocolHandler,
   nsCString mSpoofedUserAgent;
   nsCString mUserAgentOverride;
 
-#if defined(MOZ_BUILD_APP_IS_BROWSER) && !defined(ANDROID)
   nsCString mExperimentUserAgent;
-#endif  // MOZ_BUILD_APP_IS_BROWSER && !ANDROID
 
   bool mUserAgentIsDirty{true};  // true if mUserAgent should be rebuilt
   bool mAcceptLanguagesIsDirty{true};
diff --git a/toolkit/components/moz.build b/toolkit/components/moz.build
index af472c0e41d9..ca48baaed16d 100644
--- a/toolkit/components/moz.build
+++ b/toolkit/components/moz.build
@@ -129,8 +129,7 @@ if CONFIG["MOZ_WIDGET_TOOLKIT"] != "android":
 if CONFIG["MOZ_BUILD_APP"] == "browser":
     DIRS += ["normandy", "messaging-system"]
 
-if CONFIG["MOZ_BUILD_APP"] in ["browser", "comm/mail"]:
-    DIRS += ["nimbus"]
+DIRS += ["nimbus"]
 
 if CONFIG["MOZ_BACKGROUNDTASKS"]:
     DIRS += ["backgroundtasks"]
diff --git a/toolkit/components/nimbus/ExperimentAPI.jsm b/toolkit/components/nimbus/ExperimentAPI.jsm
index 475996b9f297..c28a7a7da807 100644
--- a/toolkit/components/nimbus/ExperimentAPI.jsm
+++ b/toolkit/components/nimbus/ExperimentAPI.jsm
@@ -291,11 +291,14 @@ const ExperimentAPI = {
  */
 const NimbusFeatures = {};
 for (let feature in FeatureManifest) {
-  XPCOMUtils.defineLazyGetter(
-    NimbusFeatures,
-    feature,
-    () => new _ExperimentFeature(feature)
-  );
+  XPCOMUtils.defineLazyGetter(NimbusFeatures, feature, () => {
+    if (AppConstants.MOZ_BUILD_APP !== "browser") {
+      throw new Error(
+        `NimbusFeatures.${feature} APIs are only available for browser builds.`
+      );
+    }
+    return new _ExperimentFeature(feature);
+  });
 }
 
 class _ExperimentFeature {
diff --git a/toolkit/components/nimbus/test/browser/browser.ini b/toolkit/components/nimbus/test/browser/browser.ini
index 88bf3f20776b..e1f8b3057024 100644
--- a/toolkit/components/nimbus/test/browser/browser.ini
+++ b/toolkit/components/nimbus/test/browser/browser.ini
@@ -4,6 +4,9 @@ support-files =
 prefs =
   # This turns off the update interval for fetching recipes from Remote Settings
   app.normandy.run_interval_seconds=0
+skip-if =
+  toolkit == "android"
+  appname == "thunderbird"
 
 [browser_experiment_single_feature_enrollment.js]
 [browser_remotesettingsexperimentloader_remote_defaults.js]
diff --git a/toolkit/components/nimbus/test/unit/xpcshell.ini b/toolkit/components/nimbus/test/unit/xpcshell.ini
index 36c4a91701df..f7e3276a827c 100644
--- a/toolkit/components/nimbus/test/unit/xpcshell.ini
+++ b/toolkit/components/nimbus/test/unit/xpcshell.ini
@@ -1,9 +1,12 @@
 [DEFAULT]
 head = head.js
-tags = messaging-system
+tags = nimbus
 firefox-appdir = browser
 support-files =
   reference_aboutwelcome_experiment_content.json
+skip-if =
+  toolkit == "android"
+  appname == "thunderbird"
 
 [test_ExperimentManager_context.js]
 [test_ExperimentManager_enroll.js]