What video? Do you have a menu bar showing instead of the orange button? If so, click on Tools > Clear Recent History. You can set the time range at the top, and use the Details button to see and set what will be cleared.

XP defaults to the Menu Bar instead of the Firefox button, but you can change that. See - https://support.mozilla.org/en-US/kb/common-questions-after-upgrading-firefox-36#w_how-do-i-get-the-orange-firefox-button

If you have the menu bar visible then you can go to the Tools menu to get Clear Recent History.

• https://support.mozilla.org/kb/Clear+Recent+History

You can toggle the History sidebar on/off via "View > Sidebars > History" or Ctrl+H

You can press the F10 key or tap the Alt key to bring up the "Menu Bar" temporarily if you need access to the hidden menu bar

See also:

• https://support.mozilla.org/kb/Menu+bar+is+missing
• https://support.mozilla.org/kb/how-do-i-get-firefox-button
• https://support.mozilla.org/kb/why-are-tabs-top