We are migrating from wptdriver to wptagent. Firefox is experiencing very slow SSL negotiation times and the wptagent results are significantly slower than wptdriver. This does not seem to impact chrome.
I have tried various versions of Firefox from 58 - 61, so I suspect this is an issue with the agent and not the browser.
The public agents seem to have a similar issue. Here is a firefox result with the slow negotiation times:
Here is a chrome result set which does not have lengthy SSL connection times:
I suspect this may not be fixed?