"Clear the Cache" can be found here:

• Firefox > Preferences > Advanced > Network > Offline Storage (Cache): "Clear Now"

See also Delete browsing, search and download history on Firefox

Thanks! I don't see the need for it to be in the Advanced tab area! No wonder I couldn't find it.

You're welcome

Clearing the cache has been in the Advanced tab since Firefox 2.
Older versions had it in Privacy.
In Privacy you now only have the settings for clearing private data when you close Firefox.
You can also clear the cache via Tools > Clear Recent History or Firefox > History > Clear Recent History.

I'm running Mac OS X.4.11, just upgraded to Firefox 3.6.19. Is there still a way to set closing Firefox to offer all private data to be separately selected for clearing? The Privacy tab only seems to offer immediate clearing and needs pro-active selection.

Immediate clearing is in Tools > Clear Recent History

What you select in "Firefox > Preferences > Privacy > Clear history when Firefox closes > Settings" is what is done each time you close the Firefox application (Firefox > Quit)