Re: How can I block the private browsing option on my employee's PCs?
Sorry, the Mozilla didn't include a method of turning off the Private Browsing feature. There's a couple of hacks floating around this forum, but deleting the main file that controls PB affects other functions in Firefox and editing that file might have to be done after a minor version security update - especially if that file is replaced as part of the update. (Both have been mentioned by users in the same "hack" threads.)
I'm not sure what you want to accomplish by turning it off, but if you want to be able to view the Firefox browser history on each PC keep in mind that a user can easily clear the "last hour", last "two hours", last "four hours", "today", or everything even without the Private Browsing feature.
If all your PC's are connected to the internet through a network server you might want to look into internet monitoring software, which many times is included or optional with network Firewalls. That software usually has some filtering features where you can block specific URL's and IP addresses from being accessible from anywhere on your intranet. Many companies block web sites like Facebook, Twitter, and YouTube which can be real company time wasters. Routers can also be used to block specific URL's / IP addresses from being accessed.
You can also use the HOSTS file on each PC to block URL's / IP addresses as long as individual users don't have access to the "system files".