After much frustration I may have found a permanent solution to the issue mentioned in this thread [webmasterworld.com].
Lately FireFox would hesitate when scrolling, mouseovers hesitate, it would hiccup every ten seconds or so.
Chased all the usual suspects, most of which led to possible memory issues, plugins, etc., and none of that seemed to work. I had resigned to just restarting the $%$^ thing on a regular basis, but stumbled across a thread from 2008 that seems to be working.
I'm beginning to think the problem was related to cookies and LSO's:
Flash-cookies (Local Shared Objects, LSO) are pieces of information placed on your computer by a Flash plugin. Those Super-Cookies are placed in central system folders and so protected from deletion. They are frequently used like standard browser cookies. Although their thread potential is much higher as of conventional cookies, only few users began to take notice of them. It is of frequent occurrence that -after a time- hundreds of those Flash-cookies reside in special folders. And they won't be deleted - never.
After installing the BetterPrivacy [addons.mozilla.org]
add on . . . the hesitations have virtually stopped. As I type this, FF is consuming 183,324 K of memory, but there's no hint of hesitation so I am beginning to think it was never a memory issue to begin with.
What this plugin does is clears all LSO's on exit, but doesn't appear to affect any cookies I want to keep, for example login cookies to this site. You can configure it to ignore some LSO's if it creates a problem.
If it comes back will return to report, but it's been running for a couple days without the hiccup. Currently using the latest, 3.6.8.