If you do not see the string browser.cache.disk.parent_directory, then right click in anywhere on that screen > New > string > type browser.cache.disk.parent_directory > set directory as what you want (in my case /tmp)
I downgraded without problem to firefox 3.x on openSUSE 11.4. It was not there! I tried Windows Firefox through wine. Again, It was not there!
When you get in about:config, and you type “browser.cache.disk.parent_directory”, you get no result. Well, I had no result. So to configure this setting, you must add the entry for this setting to exist.