--- a/testing/web-platform/tests/webdriver/tests/support/fixtures.py
+++ b/testing/web-platform/tests/webdriver/tests/support/fixtures.py
@@ -46,16 +46,23 @@ def _restore_normal_window_state(session
     if state == "maximized":
         session.window.maximize()
     elif state == "minimized":
         session.window.minimize()
     elif state == "fullscreen":
         session.window.fullscreen()
 
 
+def _exit_fullscreen(session):
+    """If the window is in fullscreen state, exit fullscreen."""
+    if session.window.fullscreened:
+        session.window.fullscreen()
+    assert not session.window.fullscreened
+
+
 def _restore_windows(session):
     """Closes superfluous windows opened by the test without ending
     the session implicitly by closing the last window.
     """
     current_window = session.window_handle
 
     for window in _windows(session, exclude=[current_window]):
         session.window_handle = window
@@ -150,16 +157,17 @@ def session(configuration, request):
             raise
 
     # finalisers are popped off a stack,
     # making their ordering reverse
     request.addfinalizer(lambda: _switch_to_top_level_browsing_context(_current_session))
     request.addfinalizer(lambda: _restore_normal_window_state(_current_session))
     request.addfinalizer(lambda: _restore_windows(_current_session))
     request.addfinalizer(lambda: _dismiss_user_prompts(_current_session))
+    request.addfinalizer(lambda: _exit_fullscreen(_current_session))
     request.addfinalizer(lambda: _ensure_valid_window(_current_session))
 
     return _current_session
 
 
 def new_session(configuration, request):
     """Return a factory function that will attempt to start a session with a given body.