OK, I have an old mac running Firefox. On the "Write something" response box there are three little icons on the upper right. The middle one is an "A"- "advanced editor". Click on that you'll see more icons on two rows of toolbars (at least I see this). On the lower bunch of icons, last in a box of four is a little "strip of film" icon. Click on that and the "insert video clip" pops up. Cut and copy the URL of the YouTube clip you want to share, click "save" in the lower right corner, and you'll be back in your comment box, now with a video (in html or whatever, code of some sort- it'll appear when you hit "post".