How do you password protect Firefox? You can keep your data private using a feature called Firefox Sync. Firefox Sync stores your data in a digital 'safe' on your computer and the Firefox server. The only way you can access the safe and your data is by using a key that's generated for you. There's a mobile version for smart phones etc. also.
There are also other ways to go about password protecting any program on Windows, including Firefox. I recommend creating a unique login for every Windows user. To find out how to create a user profile, Google 'create user profile for windows ____'. This will automatically protect your private data from others and allow you to set limitations on other profiles.
Below are several other ways to password protect any program on Windows including Firefox.
Password Protect folders (password protecting a folder limits access to any files or programs installed to that folder.)
Before you password protect Firefox, consider using Chrome. Chrome allows you to login to you Google account and all your user browsing data is stored privately online.