Type about:customizing<enter> in the address bar. In the new window, look for the icon. When you find it, hold down the left button on it, and move it to where you want it to be. You can move any of the icons you want from here, but some icons are locked in place.

There is also a Restore Defaults button at the bottom middle.

The missing icon probably was added by an add-on, because Firefox doesn't have a built-in video downloading button.

You can view your extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then check on the right side for anything related to video downloading and see whether it is enabled. Note: disabled extensions are at the bottom.

You also could check for updates. Above the list, there is a button with a gear/cog/wheel icon. Click that, then "Check for Updates".

Any luck?