Hello there! If you don't know your password and somebody else has it, we highly recommend you change it. Changing your password will also assist you in signing in. Please follow the following steps to reset your password (As you can't sign in, we'll be doing this form the login page):

1. Head over to the Firefox login page
2. After entering your Email (Or, without entering your email as this depends on the product), click on "Forgot password?"
3. Enter your email
4. Press "Begin reset"
5. Check your email which was used to set up the Firefox account from a message from Firefox. Be sure to check your spam folders as well! (Emails may take up to 15 minutes to appear depending on traffic, maintenance and your/our wireless connection)
6. Click on the link/button in the Email which allows you to reset your password.
7. Follow the steps there to reset your password (I can't reset mine to test as I may loose some important data. However, the steps from this one is should be pretty straightforward. You may just need to enter your new password)

Thank you for reading this whole message and be sure to report back to us if this has worked.

You could open the Password Manager. Look up the website. Look for the password field and press the 'eye' next to it.