--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -6306,17 +6306,17 @@ FullscreenTransitionTask::Run()
     // should rarely happen, it probably isn't worth to fix. Hence we
     // simply add a timeout here to ensure we never hang forever.
     // In addition, if the page is complicated or the machine is less
     // powerful, layout could take a long time, in which case, staying
     // in black screen for that long could hurt user experience even
     // more than exposing an intermediate state.
     mTimer = do_CreateInstance(NS_TIMER_CONTRACTID);
     uint32_t timeout =
-      Preferences::GetUint("full-screen-api.transition.timeout", 500);
+      Preferences::GetUint("full-screen-api.transition.timeout", 1000);
     mTimer->Init(observer, timeout, nsITimer::TYPE_ONE_SHOT);
   } else if (stage == eAfterToggle) {
     Telemetry::AccumulateTimeDelta(Telemetry::FULLSCREEN_TRANSITION_BLACK_MS,
                                    mFullscreenChangeStartTime);
     mWidget->PerformFullscreenTransition(nsIWidget::eAfterFullscreenToggle,
                                          mDuration.mFadeOut, mTransitionData,
                                          this);
   } else if (stage == eEnd) {

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -4639,17 +4639,17 @@ pref("full-screen-api.unprefix.enabled",
 pref("full-screen-api.unprefix.enabled", true);
 #endif
 pref("full-screen-api.allow-trusted-requests-only", true);
 pref("full-screen-api.pointer-lock.enabled", true);
 // transition duration of fade-to-black and fade-from-black, unit: ms
 pref("full-screen-api.transition-duration.enter", "200 200");
 pref("full-screen-api.transition-duration.leave", "200 200");
 // timeout for black screen in fullscreen transition, unit: ms
-pref("full-screen-api.transition.timeout", 500);
+pref("full-screen-api.transition.timeout", 1000);
 // time for the warning box stays on the screen before sliding out, unit: ms
 pref("full-screen-api.warning.timeout", 3000);
 // delay for the warning box to show when pointer stays on the top, unit: ms
 pref("full-screen-api.warning.delay", 500);
 
 // DOM idle observers API
 pref("dom.idle-observers-api.enabled", true);