On Firefox, command blocks cannot be dragged if the block is grabbed in the padding or border of the block. The block must be grabbed at the button content.
The command blocks are implemented as HTML buttons and there is a bug in Firefox relating to the dragging of buttons: https://bugzilla.mozilla.org/show_bug.cgi?id=568313
Original Jira issue: https://fluidproject.atlassian.net/browse/C2LC-188