browser.cache.disk_cache_ssl should be set to "false"
Verified with TorButton v1.4.5.1 and TorBrowser v2.2.35-3 (Windows XP SP3)
The about:config setting "browser.cache.disk_cache_ssl" [1] is set to true in TorBrowser, even after changing TorButton config to "Block disk and memory cache access during Tor" [2] and toggling Tor status (via. TorButton). Mozilla suggests that about:config setting to be "false" by default [1].
I think this isn't such a big issue because TorButton does set both "network.http.use-cache" [3] and "browser.cache.disk.enable" [4] to false, which should mean "browser.cache.disk_cache_ssl" won't cache to disk, but even Mozilla seems to suggest it's wise to set all three to "false".
[1] http://kb.mozillazine.org/Browser.cache.disk_cache_ssl
[2] Note: Should the text not read: "... cache access during Tor use" vs. "... cache access during Tor"?