To get OneClick Youtube Downloader icon working for Firefox 5, here’s how:
Go to www.orbitdownloader.com/, or the File Hippo page for Orbit Downloader and download the latest version of Orbit (Orbit was already on my computer, but I reinstalled it using the latest version), which is 184.108.40.206 (as of writing; later versions will work with FF5 too).
Restart Firefox a couple of times … and there it is, back in the toolbar, and working as always.
(Of course, you need the Firefox add-on OneClick YouTube Downloader, now version 1.0.7. Once you download and install the latest Orbit, this extension should be in your Firefox add-ons. If the icon is not on your toolbar, right click in a toolbar, select ‘Customize’, then drag-and-drop the Recorder icon onto your toolbar).
EDIT to add: On the Orbit website (link above) they list their latest version (as of writing) as 220.127.116.11. On the File Hippo page for Orbit Downloader they list the latest version as 4.1.2. These are both the same, and either download source is good.