Comment 389 for bug 195698

This option shouldn't be considered hidden, because it's accessible from the menu system :).
There are lots of others which require some knowledge of mozilla internals.
I'll give some shortcuts below as examples:

1. The "Security Devices" dialog has an url (you can open it directly or add it to your bookmarks):
chrome://pippki/content/device_manager.xul

2. You can also use some javascript (open Tools > Error Console [Ctrl+Shift+J], paste the following line of text in the Code box and press Enter or click Evaluate):
Components.classes["@mozilla.org/security/pk11tokendb;1"].getService(Components.interfaces.nsIPK11TokenDB).findTokenByName("").logoutSimple();
(Note: There will be no feedback, so you should check that you're really logged out. You can do so with almost the same command, substituting isLoggedIn for logoutSimple.)

3. If you plan to use it frequently, you may create a firefox extension that adds it to the tools menu and gives it a shortcut key.
It's a bit off-topic and about 30 lines of code, so I've put it here: http://pastebin.com/f7cd21c99 (should I create an attachment?)

+1. In some circumstances you may use Tools > Clear Private Data [Ctrl+Shift+Del], ticking only "Authenticated Sessions". This does a bit more to logging out from the software security device, but makes sense to use when letting others to your computer.