If you've got saved log-in data you can find it as follows:
Click the 'hmburger'. (Three lines, top right)
Click 'Options'
Click 'Security'
Click 'Saved Logins'
You can then delete anything you don't want by clicking on it and then on 'Remove'.
If the data you're seeing isn't log-in information, but simply auto-fill form data, click in a field where you're seeing things automatically filled in. Then press then 'down' arrow to see all of the options which are offered to you. Use your mouse, or the 'down' arrow again, to select an entry you don't want and press 'Delete'.
If you want to clear ALL form data:
Click the hamburger.
Click 'History'
Click 'Clear recent history'
Select 'Everything' in the 'Time range to clear' field
Click on the arrow next to 'Details' to see what can be cleared
Ensure that 'Form & Search History' is ticked
Click 'Clear'
To prevent Firefox from storing any form data:
Click the hamburger
Click 'Options'
Click 'Privacy'
Set 'Firefox will' to 'Use custom settings for history'
Remove the tick next to 'Remember search and form history'