You can't do that reliably in Firefox. If you would use an extension then holding down the Shift key to start in Safe Mode is sufficient to bypass the password. Best is to use a separate Windows user or guest account and protect your Windows account with a password.

There may be something here, but I doubt it.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.