Over time Firefox accumulates a lot of cruft in its autocomplete list. If you want to delete them, here's how (from support.mozilla.com/en-US/kb/Form+autocomplete).
If you want to remove one of your previous form entries from Firefox's history:
- Click on the form field and press the down arrow key to display all the saved entries. You may type the first few letters of the entry to limit the number of entries displayed.
- Use the down arrow key or the mouse pointer to highlight the entry you wish to delete.
- Press Shift+Delete. The entry will be removed.
- You may repeat the process for any number of entries for a particular form field.
If you want Firefox to forget ALL of your previous form entries, you need to clearing form history:
- At the top of the Firefox windowOn the menu bar, click on the Tools menu, and select Clear Recent History.... A dialog box will appear.
- In the dialog box, set Time range to clear: to Everything.
- Click on the down arrow beside Details, make sure only Form & Search History is check marked.
- Click Clear Now. The dialog box will disappear and your form history will be erased.