For most users we can recommend the program Exe Password which works on any of the existing versions of Windows. Download it on the official website of the developers. The download link you will find in the Download section. After the installation file is downloaded, run it and install the application.
Now click on the Opera shortcut, right-click and select the context menu item Password Protection, which appeared after installing the program. You will see the setup Wizard password. Enter the password in the New Password field and retype it in the Retype New field to P. Press the Next button and then Finish button. Start Mozilla and verify that the password is installed and run the program requires its input.