Prevent Firefox from storing form entries
If you don't want Firefox to remember what you've entered into form fields, you can turn off the auto form fill feature:
Click the menu button and choose Options.Preferences.
Select the PrivacyPrivacy & Security panel.
In the drop-down menu next to Firefox will:Firefox will, choose Use custom settings for history.
Remove the check mark next to Remember search and form history.
Close the about:preferences page. Any changes you've made will automatically be saved.
Disabling form history also prevents Firefox from storing search history for the Search bar in the Navigation Toolbar.