While the saved session data is stored locally on your computer, the files can contain personal data depending on what web pages are open when a session is stored. In addition some web sites store all the login information in session cookies which will be saved by Session Manager, unless the option to do so is disabled. This can potentially allow anyone who loads that session to automatically log in to your account on those sites. As such it is a good idea to safe-guard your session data.

If you share the computer with others and wish to protect your session data, enable encryption in the Session Manager Options, Advanced tab. For even more protection, enable the master password in Firefox.