This question has been solved by Firefox Help but perhaps the solution may help the many non-techies like myself who are plagued by these adwares. After browsing a moneysaving 'coupon off' website my new Firefox 57 slowed to a crawl. I tried the usual restarts and cleaning but then noticed that two cookies KRDX.NET and businessinsider were returning a few seconds after being deleted. Online search indicates that these are activity trackers for spamming users with targeted ads. I am guessing that the cookies were causing a search for appropriate ads which were blocked by uBlock anyway. After struggling for several hours, and AdAware, Malwarebytes and Kaspersky finding nothing amiss, I discovered Firefox's own solution which was the REFRESH button under Troubleshooting. This removed the two cookies and I can now enjoy what you rightly say is the fastest Firefox yet -- thank you!