Basic scenario is as follows.
Firefox will white out (the title bar text ends with "Not Responding" and the application is overlaid with a white screen), and Win7 eventually (or if I try to terminate) presents its alert window offering to check online for a resolution and terminate, terminate, or (sometimes) wait for the application to respond.
Occasionally, a second alert will appear stating, "Plugin Container for Firefox has stopped working," with the first two (2) of the three previous options.
There is no clear indication whether this second alert is a Firefox message or a Win7 message, but I'm inclined to believe it a Win7 alert.
There is, neither, a clear indication whether the Plugin Container is just a plugin manager, or if it includes all extensions, plugins, and themes.
Online search (Google) returns a plethora of results, but almost all are extension/plugin/theme specific, and none have yielded any hard results.
As to cache cleaners, I don't use 'em. Became allergic from the first one I used in Win95. I've tried several since then, in all iterations of Windows save for WinME, and found them to create more problems than they resolved. Some people swear by them, but in my experience, all it takes is one mis-perceived memory address cleared and a system crash is imminent.
Just as a sidebar, the problem under discussion has occurred three (3) times during the creation of this post, pretty much obviating idle time or lack of activity as a probable cause.