An easy shortcut in Firefox to clear your private data is CTRL-Shift-Delete. Try it.

Another recommendation I have, if you are using Firefox to store your passwords, consider setting a Master Password (it's in the option screen too). This way as soon as you load Firefox, it will popup and prompt you for your master password. If you get this password wrong (or click cancel - for someone else using your computer), it will not auto-fill any passwords you have saved. It uses this single master password to encrypt all your other saved passwords.