There was a failed download in my download history when I moved to the nightly containing the new panel. Completed downloads can be removed without problem, but the failed entry doesn't have an option to do so.
I assume you mean in the context menu?
Created attachment 617549 [details] [diff] [review]
Simple fix. Thanks for reporting!
Verified on the latest Nightly that the failed downloads can be removed using the item from the context menu "Remove From List".
Verified on Windows 7, Ubuntu 12.04 and Mac OS X 10.7:
Mozilla/5.0 (Windows NT 6.1; rv:19.0) Gecko/19.0 Firefox/19.0
Build ID: 20121111030749
Mozilla/5.0 (X11; Linux i686; rv:19.0) Gecko/19.0 Firefox/19.0
Build ID: 20121112030753
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:19.0) Gecko/19.0 Firefox/19.0 Build ID: 20121112030753
Setting the status of this bug to VERIFIED FIXED.