If you don't want Firefox to remember what you've entered into form fields, you can turn off the auto form fill feature:
At the top of the Firefox window, click on the Firefox button (Tools menu in Windows XP) and then click Options. Select the Privacy panel. **Set Firefox will: to Use custom settings for history. Remove the check mark from the box that says Remember search and form history.** Click OK to close the Options window. .
Disabling form history also prevents Firefox from storing search history for the Search bar in the Navigation Toolbar.