And how to remove this underlining of letters in the font?

Maybe you should password protect your O/S so no one can use your computer before accessing it. The Browser has no password protect for anyone to open and use it.

You can use the Primary Password to block access to the logins, but you can't prevent accessing Firefox.

• https://support.mozilla.org/en-US/kb/use-primary-password-protect-stored-logins

You should protect your Windows account with a password and use a Windows Guest account for others that use your computer.