If you’re using a shared device and don’t want Firefox to remember your passwords, you change your preferences in the settings:
Click the menu button 
and select .
Click in the panel to the left.
Scroll down to the Logins and Passwords section.
Uncheck the box next to Ask to save logins and passwords for websites.
Disable password saving for specific websites
You can also configure Firefox not to save passwords for specific websites. To add these exceptions:
Click the menu button and select .
Click in the panel to the left.
Scroll down to the section.
Click on next to Ask to save logins and passwords for websites.
Add the websites you don’t want Firefox to save passwords for in this panel.
Click .