Well, it seems you already have the latest update (5.4.2): https://addons.mozilla.org/firefox/addon/video-downloadhelper/ When an add-on button is missing from the toolbar, you might be able to add it back using the Customize tab. See this article for more information: [[Customize Firefox controls, buttons and toolbars]]. Can you find it on another bar, or on the overflow list, or among the unused icons and relocate it where you want it? If it's nowhere to be found, you can use the Restore Defaults button at the bottom of the Customize tab to reset the toolbars and hopefully that will straighten it out. Any luck? If not, this add-on has its own support site here where you might find the solution in an FAQ or forum post or pose a new question: http://www.downloadhelper.net/support.php