We use to have a password that was set to pop up on the opening of firefox. I recently updated firefox and can no longer find the place to do this again.
Firefox has a Master Password feature. When Firefox needs to access your stored log in information, you will be prompted for a password once per browsing session.
Create Master Password
1. Click Tools on the top of the Firefox menu bar
2. In the drop-down box, click Options
3. Click the Security Panel
4. Under Passwords, click Use a master password
5. A Change password window will pop-up. Please enter your new password twice. There will be a password quality meter to help you.
A Master Password won't stop people from opening Firefox, if that is your intention.
There is an extension that can do that, but extensions can easily bypassed by starting Firefox in Troubleshoot Firefox issues using Safe Mode
Profile Password: http://nic-nac-project.de/~kaosmos/profilepassword-en.html#PPFF