The only browser that works for posting youtube videos is Internet Explorer. I have only posted videos in blogs or inboxes. Not sure if they work the same on message boards. Go to youtubes site and choose what video you would like, then click on share as in the picture below.
Click on Embed
Click to check, use old embed code
Now select and copy the entrie code in that box
on the blog post, click on comment and you will get a screen like this, click the box where the red arrows are pointing.
now paste that code into alternative text
click anywhere in that light blue box area, then hit ok then post.