Looks like a bug.
Following this KB entry:
uploaded a .swf file on the server. However, when using the Flash Mgr in the content block, the "insert" or "delete" buttons do not show up!
Firefox 28/Windows 7 Pro/Sitefinity 6.3
Please advise if it's a bug, or if there's an update to the Flash insertion process.
Honestly I didn't try to "double-click" to insert the Flash file because we moved away from using Flash at this point. IMO, you should reinstate the buttons though since it makes the process less intuitive without them.
Thanks for your help.