Hello,

That information is usually as cookies. To clear cache and cookies do the following:

1. Click the menu button , choose History, and then Clear Recent History....
2. Under "Time range to clear", select "Everything".
3. Now, click the arrow next to Details to toggle the Details list active.
4. From the details list, check Cache and Cookies and uncheck everything else.
5. Now click the Clear now button.

Further information can be found in the Clear your cache, history and other personal information in Firefox article.

You can also browse those sites in Private Browsing Mode. Please see Private Browsing - Use Firefox without saving history for more information.

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Thank you.

Use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list with the mouse or cursor Down key
   do not click the mouse or press the Enter key
3. press the Delete key (on Mac: Shift+Delete) to delete the highlighted entry