Fix Firefox preferences to let WebSockets have a carefree childhood (#1299)

Disable FF's implementation of RFC 6455 (WebSockets) section 7.2.3 (backoff when
recovering from abnormal closure):
https://www.rfc-editor.org/rfc/rfc6455.html#section-7.2.3

While this is a good thing in the wild, during testing we want to test the
driver and not the browser, so it should get out of the way. Further, the tests
often expect/enforce failures and we don't want those tests to pollute the
environment for following tests.

While at it, also bump the browser's WS limit so that we hit the driver's max
pool size before ever hitting the browser's limit.

See also: https://bugzilla.mozilla.org/show_bug.cgi?id=711793

Author: robsdedude

testkit/firefox_profile/prefs.js

@@ -1,3 +1,5 @@
 /* global user_pref */
 user_pref('network.dnsCacheExpiration', 0);
 user_pref('network.dnsCacheExpirationGracePeriod', 0);
+user_pref('network.websocket.delay-failed-reconnects', false);
+user_pref('network.websocket.max-connections', 10000);