Some tests such as /html/select/options-length-too-large.html run their test cases as part of document loading when run with WebDriver. The default page load timeout is 300 seconds. This test uses the "long timeout" metadata tag which tries to set a timeout for scripts at around 65 seconds.

The script takes more than 65 seconds to run which the Python test harness as the browser crashing, because the WebDriver server does not report a timeout. This is due to the fact that the "page loading strategy" is normal ie the WebDriver load command finalizes after the load event (after script).

I suspect that these kind of problems often aren't an issue in other browsers because the tests simply pass.