But the question is: what is causing this abnormal CPU usage that is clearly associated with Firefox's unresponsiveness? 

For the record, I have run repeated scans using Windows Defender and MalwareBytes and nothing malicious has been detected, and I have had no other problems with my computer besides the ones with Firefox.  Chrome continues to operate perfectly normally.

UPDATE 9th Feb <br />Issue now SOLVED in the other thread.<br /> Disabling McAfee SiteAdvisor resolves the issue in this instance. [/questions/984055?page=2#answer-532036] <br />Note presence of ''McAfee MSS+ NPAPI Plugin'' Please see and continue with thread [/questions/984055] ~J99 Modified February 1, 2014 9:39:33 PM BST by John99 --------------------- Almost two weeks ago, after years of admirably flawless performance, my Firefox 26.0 was struck by recurrent episodes of complete unresponsiveness, and frequent crashes. Efforts to remedy these problems, involving disabling various add-ons and JS, failed. Now I have some important new information. When operating normally, Firefox never uses more than about 3% or, rarely 5% of my CPU, even when I have many tabs open to websites, even when several of those websites are playing videos, etc. (I have had the Task Manager open constantly this week in order to be able to close Firefox when it's so unresponsive I can't use X to close it, so I was in a position to observe CPU usage.) But when Firefox is in the early stages of an episode of unresponsiveness (for example, when I can't download files or play videos) the CPU usage is greater than 25%. What's even more remarkable is that if I then close all tabs, and have only one “New Tab” open (so that I'm theoretically not using Firefox at all), the CPU usage remains over 25%!! Over the past few days I've a) confirmed and reconfirmed this perfect correlation between Firefox's unresponsiveness and a dramatic, unprecedented increase in CPU usage to in excess of 25%, occurring completely out of the blue, in no way triggered by my doing anything, and b) verified that when Firefox is responding normally, NOTHING I do causes the usage to go above about 5%. But the question is: what is causing this abnormal CPU usage that is clearly associated with Firefox's unresponsiveness? For the record, I have run repeated scans using Windows Defender and MalwareBytes and nothing malicious has been detected, and I have had no other problems with my computer besides the ones with Firefox. Chrome continues to operate perfectly normally.