Random switch_window firefox failures (W3C)

With the switch to W3C webdrivers @ MDL-66979 it has been detected that there are various @_switch_window scenarios that are failing with Firefox.

They don't happen always, we have some runs passing. And they are hard to reproduce locally, only as big runs @ CI (accumulative effect?).

Both Boost and Classic themes affected.

Links:

• Boost: https://ci.moodle.org/job/W39.04.01%20-%20Behat%20-%20Firefox%20+%20Postgres/145/
• Classic: https://ci.moodle.org/job/W311.07.01%20-%20Behat%20-%20Firefox%20+%20Postgres%20+%20Classic/54/

Error (for example, there are many, any @_switch_window):

001 Scenario: Add a preconfigured tool from a cartridge # /var/www/html/mod/lti/tests/behat/addtype.feature:37

      And I switch to "add_tool" window                 # /var/www/html/mod/lti/tests/behat/addtype.feature:45

        Unable to locate window: add_tool (Facebook\WebDriver\Exception\NoSuchWindowException)