This URL does not exist anymore in ESR45.

Also, stop using the FirefoxTestCase class from marionette/firefox-ui-tests.

This should have been removed by commit fed6ce1d.

The user timing API testing was initialy added to the resource-timing
test. For better clarity, we move it to a separate user-timing test.

On Windows the paths in PYTHONPATH are separated by ';' and not ':'.

Following changes from #18557

The bridge used by this test is currently offline.

When strace is used, the browser get stuck in a loop while running the
mozmill tests. We disable the use of strace until this is fixed.